This is a good question. Here are a few typical reasons why later-stage companies join YC. 1) Companies that are in international markets and want to launch

This is a good question. Here are a few typical reasons why later-stage companies join YC. 1) Companies that are in international markets and want to launch in the US. YC can provide a great platform for this. 2) Companies that sell services to startups (i.e., devtools) and can make use of YC's network to get customers faster. 3) Companies that have bootstrapped thus far but now want to raise a larger round from investors and want help doing so. 4) Companies whose business is at a substantial size but not growing quickly, and want to grow faster. 5) Companies that want to make a major business change, and want help with doing that. We've had companies pivoting from hardware to software, from a service company to a product company and more.