people who specialise in one thing tend to get very good at it. for example, there are graphic designers who do nothing but book covers and are intimately familiar with that visual language and how to apply it effectively.

you almost always want to hire a specialist like that if you want something done on a professional level.

on my part, i know a little about a lot. jack of all trades, master of none, with the exception of software development, which i did for a living until i burned out 3 years ago.