1.) Putting money into the corporate bank and then taking it out as a nominal salary keeps you really honest with yourself. You can't say "There's money in my bank account, there's not much need for urgency", instead you see the corporate bank account dribbling dry just as if it were someone else you were paying.
2.) It reduces personal risk in case the startup goes bankrupt.
3.) It makes accounting & record-keeping easier for when the startup actually does start making money, since you already have systems setup.
4.) Minimum wage laws require it. I talked to a (Massachusetts-based) lawyer once that suggested this may be an issue. But then, if it is, how does that square with all the tech company CEOs that take $1 salaries?
5.) It lets you take advantage of certain personal financial products that are only available if you have earned income in a year, eg. Roth IRAs.