The purpose of these big rounds, usually, is founder liquidity.

Snapchat could easily get acquired, making the founders (and early employees) very rich.

But investors don't want them to sell yet. So investors buy a lot of common stock (from founders/employees) at a big valuation, making the founders (aka the only board members who aren't already rich) rich, so that the founders are less interested in selling.

Some of the money goes to the company. But it's not usual for 20-25% of big rounds to go to founders/early employees.