After looking at other tools and thinking about your user, putting what you've learned and what your goals are for your design into writing is important. A short summary will help you:
cement what you've observed and learned
remind yourself of the ultimate goal for the tool
remind PIs, coworkers, etc of the ultimate goal for the tool
help you to define core features and the minimum viable product (MVP)
In addition to writing up a short summary, it can be surprisingly helpful to write up the most plausible actions/stories that someone would take while using your tool. This is called a user story and will help you make sure that you meet the needs of your most common use case.