>why would I want to work specific muscles

As a beginner, you would not. You want to work the whole system, and to do that, you only need a handful of compound movements: squat, deadlift, and overhead press.

Once you slow down in progress on these bigger compounds, that's when you would want to start doing more targeted or more specific assistance exercises, in addition to the compounds.

Starting Strength is a good enough book to learn this stuff. Read it and beware advice on the subject of fitness, for every nerd, bro, and their brother gets a kick out of speaking with authority on this stuff, even if they don't know what they're talking about - just go straight to a reliable source: Starting Strength. It's become a meme, but it's good enough.