[7stud]

The forced bring in effectively increases the amount of the antes. The higher the antes are in relation to the average future bets, the more agressive the game gets. That's because as the antes get larger relative to the bets, you are forced to contest the pot with less than premium hands: if you only waited for premium hands the antes and your share of the forced bring ins would be more than the pots you won.

Then, I guess the question becomes, why not just increase the ante a little bit for everyone instead?

Having someone bring it, initiates the action immediately, and having the low card bring it in stimulates the action by allowing higher hands to hide their strength, and allowing the high card to fold.