Suppose you haven't released your app yet, or you have major changes to make to make your app as successful as it might be. You might have some revenue or none, but you probably won't have enough to hire 3 full-time developers (as an example).
Ah I see, never thought about hiring people. I ask because I'm working on an idea too (just released beta), but doing all the work myself after my full time job. I guess doing it full time would be best case scenario, but then again getting involved with investors can be a huge pain ( used to work at a startup, it did not end well ).
Whether investors are a good idea depends on your product, customers, market, and yourself. However, I'd recommend YC unequivocally.