On the most basic level, you need to understand how people are using your product. Without Amplitude (or another analytics tool), it would be very difficult to grow.
I use this for so many things. Acting as a short-term API, with its ability to produce JSON. Using it to collect form data (I used this on TheCounterpart.co). Connecting it with GA and Optimizely to report on tests/analytics automatically and recording data. It's my go to for so many things.