You might not want to work specific muscles. But if you did, it could be for as basic a reason as vanity or just thinking having large delts or biceps is "cool". Mike Israetel has a good video on the general principles that go into specialization:
https://youtu.be/BsuI-E_iIWc However, if you're a beginner, you should not specialize, in my opinion. Being completely untrained, strength and hypertrophy gains are so easy to come by — you can progress linearly for quite some time before reaching even an intermediate level. You would want to focus on big, compound movements which involve muscle groups and then potentially layer on top of that some isolation movements (usually at higher reps, possibly more sets), purely for hypertrophy. The truth is, though, that even the compound movements do target muscle groups though(groups, not a single muscle), e.g. a bench press will work pecs, delts, triceps and others, whereas the pecs are minimally involved in deadlifting and, instead, they work the glutes, quads, the posterior chain generally, etc.
I'm rambling a bit, but the point is that compound movements are so effective and they're really the most efficient way to drive strength and hypertrophy gains, especially as a noob, that it wouldn't make sense to do a ton of isolation movements. And even if you did do a bunch of isolation (as opposed to compound lifts), it wouldn't make sense to just pick one muscle while minimizing others while you're still building up a base of strength and physique.