I don't work in a startup but the culture at my place is very startup like. Lot of prototyping and fast iteration. In such a fast moving environment, code is constantly evolving so without the right documentation, it can get extremely hard to understand what the code is even trying to do. Of course there's a tradeoff between actually getting work done vs documenting what you're doing. But having the right level of documentation is a great investment since it allows people to actually focus on moving forward with development.