What's the point of contributing to the camp funds?

It can increase your honor.

The reason that this helps is self-explanatory. Some people in your gang will be more positive to you, you can get discounts on a lot of restaurants, and you will have a lot of benefits from people (apart from enemy gangs like the O' Driscolls, of course).

In conclusion, it increases your honor, which gives a lot of benefits.

In addition to increasing your honor by donating to camp funds, it is also one of the ways to fence stolen or valuable gear.

While the Fence at Emerald Ranch is unlocked pretty early during the Chapter II quest "The Spines of America". If you have Gold Watches and Silver Belt Buckles you need to offload before completing this quest donating is the only way to do so. Even after unlocking the Fence it may be more convenient to donate to the camp rather than make the trip to Emerald Ranch.

Of course, donating to the camp means the funds can only be used for camp upgrades. Still, I find this to be a good way to boost camp quality immediately after starting the game.