New features, atleast for us. The first set of features were easy to implement (to create the minimum viable product), but then things got more and more complicated. The old code base was slowly deteriorating due to lack of future proofing (ha! as if there is such a thing in programming), and we needed to rewrite large chunks of code. This was slowly consuming everyones time, and we were slowing down. Hiring more engineers gave us more man hours to continue to make the product better, while keeping the codebase tidy and maintainable.