It alerts you to important things.

As a feral druid, I need to have savage roar up at all times. I set it up to blink rapidly and annoyingly when my savage roar had 2> seconds left.

If you are a shaman, it can tell you when your shields are running low, when to recast your fire totem, ect.

Best addon ever, IMO

So what can it do that MSBT can't?

@Eregos ftw! There are many many addons that provide the function you describe, and do it better. So again, the users of Power Auras have yet to provide a good reason to use it over something else.

I would suggest that some of the advantages to Power Auras over MSBT or the like would be:

A) The information can be persistent on screen as long as appropriate, particularly useful for reactive, on next strike, or mid-length cool down abilities.
B) Power Auras can be used to track things you're missing; MSBT will have no way of telling you 'Dear Paladin, you fail at having a blessing on', Power Auras can.
C) Power Auras has timers that can be used to show the duration of proc effects.

@wow

Addons are a personal preference. I've been born and raised with Power Aura's, and I find it simple to set up. It works for me. If you have another addon that does the same thing, and you find it easy to set-up, by all means stick with it. Don't switch to PA just because everyone else is using it.

Take bigwigs for example. It's a great addon that works just as well as DBM, but you are laughed at for not having DBM cause bigwigs works for you.

MSBT has a "skill is ready!" component. This is entirely different.

First of all, Power Auras uses graphics to tell you information. People respond better to images. It takes some getting used to, but you configure all the images yourself, so it's not too difficult. You can even just utilize the original spell graphic.

Second of all, Power Auras has triggers for things that you wouldn't even believe, only limited by your own imagination.

For my Shaman, it's important for me to know that my Flametongue Weapon buff has fallen off, so I chose to plant a big mostly-opaque red circle dead-center on the screen, indicating that I am currently shorting myself 274 spellpower. Similarly, I have a blue circle in the same area indicating that I am shorting myself 47 / 172 spellpower via Flask of the North / Frost Wyrm. These are large and annoying persistent circles on the screen for as long as I stay in combat.

For my Paladin, it's very easy to run out of mana if I can't keep Divine Plea up, so I set up a system that shows me the timer on my current Divine Plea. If that timer expires, a large graphic shows up and stays up until I use Divine Plea again.

While I was leveling my mage and my hunter, I used one that would tell me exactly when Deep Freeze / Kill Shot were ready. It's difficult to know exactly when 20% hits or if the ability is still on cooldown from a previous kill while questing, so this only shows up if the skill is off cooldown and if the enemy is below 20%. As for Deep Freeze, I did some dungeons while leveling that mage and it was incredibly useful to know when it was ready to use, because it's hard to tell when you've "frozen" an enemy that cannot be frozen.

On my priest, I merely have a bunch of spell icons for the abilities with short cooldowns I use all the time during boss fights depending on the spec (CoH, PoM, Penance), and dimming the icon while it's not ready, showing a timer above when it will be.

Typically while leveling any class, I will use large buff icons in the top-right (I don't keep buffs there any longer as I use Elkano's Buff Bars along the right edge) that indicate when an important class ability needs to be re-buffed, because it's sometimes difficult to keep track of time when you're quest grinding and you can forget that you haven't buffed yourself for hours.

To sum it all up, Power Auras is useful for:

1) A reminder to buff yourself or others.
2) Tracking procs that change your priorities.
3) Showing timers for how long skills have until they're ready or how long a debuff will last on the target or how long a buff will last on you.

The best thing about power auras is the persistence and graphical nature of the display. The best example is keeping track of mage arcane blast stacks. I have it set up on mine to make a purple quarter-circle around my character for each blast stack. That way, I can see how many stacks I have up and just know it, without having to look at an icon or a buff bar. It ends up feeling a lot more natural than any other way I can imagine, in terms of tracking that type of a buff. (Rich Maloy talked about his setup with Maelstrom Weapon stacks.)

I personally prefer auditory alerts for procs (like missile barrage), which MSBT handles just as well as Power Auras.

It sounds like Power Auras does nothing that is useful to me, then. MSBT has triggers that I can use (yes, it can do more than alert when a skill is ready) and my buff frames already track my buffs. On-screen notifications (especially anything that stays on the screen for more than 20 frames) are far more cumbersome than audio alerts.

@Avan
Instead of dismissing Power Auras, why not try it out. Although you seem to have already made up your mind that your way is best, so on second thoughts don't.

@ Avan. You can create customizable audio alerts with Power Auras.

Rather than cast judgment on Power Auras without even using it, why not download it and give it a try? I had very little knowledge of PA when I first began hearing all the raves about it. I decided to download it and see what all the fuss was about. I haven't looked back since, and I can say without hesitation that it's the greatest addon ever.

@uncaringbear:

Both MSBT and my preferred buff frames can do audio alerts. At this point, there really isn't anything new that Power Auras could bring to my UI that isn't already there.

To those suggesting I just try the addon:
Why? So I can set it up improperly in a non-working manner and then deem that it sucks? Is there something bad about getting the scoop from people who actually know how to use it that I'm not aware of?