Aside from making you feel like a big man, these businesses have a very tangible benefit: cash. As each day goes by, you'll automatically rake in cash from your holdings, starting with a few hundred and ramping up to tens of thousands of dollars. Complete the simple missions that come with each of the six racket types (protection, drugs, prostitution, etc.), and you'll earn even more.
There's a catch, though: your rivals won't appreciate your muscling in, and so you'll have to defend your businesses against random, seemingly constant attacks. This gets really tedious, especially when a racket on the other side of town suddenly gets attacked while you're in the middle of something else. The good news is that you can usually ignore it - a business can take one or two raids before it's repaired or lost - but it's still a huge pain.
Rivals aside, there are a few silver linings; not only do your rackets enable you to recruit a few armed underlings to follow you around and watch your back, but all that money will make it easier to take advantage of the new "bribe" feature. If you're killed or arrested, you'll now have the option of paying $2,000 to get all your cool, expensive weapons back. Whether you think this is cheating or long overdue, it takes away a lot of the frustration that comes with failure in GTA. Which is great, because you'll feel that frustration a lot over the course of the game.