What is the purpose of activating a VFR flight plan?

Filing versus activating

Filing a VFR flight plan with Flight Service doesn't do anything by itself. You have to activate it when you depart. A plan that's filed but never opened just expires, and nobody comes looking.

What happens if you don't close it

Once it's active, the plan is your safety net. If you don't close or extend it within 30 minutes of your ETA, Flight Service starts search and rescue (AIM 5-1-14). The plan tells searchers your route, your fuel on board and who's with you.

No rule requires one

For a domestic VFR flight, no rule requires a flight plan, over water or at any distance. A flight across Long Island Sound or Lake Michigan needs none. The exceptions come from other rules. Crossing an Air Defense Identification Zone requires one (14 CFR 99.11), and so do international flights, such as a trip to the Bahamas (91.703). The 250 nm figure is from the instrument rating cross-country in 61.65, which is flown on an IFR flight plan.

ATC doesn't see it

A VFR flight plan is filed with Flight Service and stays there. ATC can call and ask to see it, but by default they don't receive VFR flight plans, and they learn your intentions only when you call them.

Flight following isn't the same

Flight following is close, but not quite. ATC only provides it workload permitting, and many pilots cancel it in the air once the airport is in sight. That stops anyone from watching the flight. If you have an incident during landing, it could be several hours before someone notices.