I'm not from PlanGrid, just discovered the tool and my co-workers are running some tests with future projects. However, the answers to your questions (From my POV):
- ELI5 what "free for 50 sheets" mean : This means that you can upload 50 PDFs that contain your plans (Blue Prints). So if you are undergoing house renovations and your Architect drew up plans that are 25 pages, then half of your quota would be used.
- Is there any benefit in using it for small, private projects : That depends. If you're outsourcing your work and have a contractor, then yes. The plans are initially updated on the service and any revisions that occur throughout the project would be instantly synced.
PlanGrid, from my limited exposure, is a markup tool for the initial plans. Their goal is to keep everyone in the "know" with a project that constantly changing. If you and a buddy are working on a deck, might not be useful. If you are contracting/sub-contracting work, it will be very useful.
My 2 cents.