Why do you need to provide your arrival airport when you close a VFR flight plan?
My understanding is that providing multiple pieces of information related to your flight plan when communicating is to ensure that the correct flight plan is being manipulated.

It seems odd that it would be required on an electronic format, but what if your N-number is N1234RF and a person closing the flight plan types N1234EF?

There is potential to close the wrong flight plan.  Now the likelihood of both of those N-numbers having an active flight plan to the same destination and having someone fat-finger it at the same time is much less likely, obviously.

Also, "if you are flying to multiple airports this function provides a chance to confirm which of the flight plans you are using. Ex. a student pilot flying from their Home airport to airport A, taking a break then flying onto airport B before flying back home." - Dawn Breaker

So it is just a cross-reference for accuracy.