Macros are all about saving clicks and key presses. Next to addons, they are probably the largest quality of life enhancement you can add to your gameplay. For the uninitiated, macros are simple scripts that allow you to combine multiple abilities and conditional parameters into a single button. You don't need them to play, but the benefits are tangible and will improve your overall performance.
If you're an engineer, take Synapse Springs for instance. It's off the global cooldown, which means you can stack it on top of other abilities to maximize its uptime and not worry about remembering to use it. Sometimes certain abilities will line up closely with other cooldowns which makes it beneficial to macro it in instead of using autocast. Have you ever noticed how slowly your pet will switch targets sometimes? Or when you tell it to Kill Command on a new target, but it attacks its previous target instead? There's a way to fix it with macros. As a keyboard abuser, one of my favorite macro perks is being able to remove the error messages and "spell not ready yet" sounds my character makes as I spam my buttons.