Here in Idaho we have a lot of mountains and desert areas where if I had to make an emergency landing or crashed it may take a very long time to find me. In that case I can see the benefit to opening a flight plan. But in areas where I'm flying over most or all populated areas, someone would see the crash well before they would start looking for me. In that case, are there other reasons to file and open a flight plan?
According to the AIM section 5-1-4, the main reason is search and rescue:
b. It is strongly recommended that a flight plan (for a VFR flight) be filed with an FAA FSS. This will ensure that you receive VFR Search and Rescue Protection.
So the question becomes, does search and rescue have value in populated areas? I would say that - excluding large cities - populated areas are actually not very densely populated at all (relatively speaking), and there is a very good chance that no one will see your aircraft come down or even react if they do. You might be flying at night, or no one may be looking in the right direction, or they may assume you're landing at a private strip or doing cropdusting or whatever.
And even when an aircraft is known to be down in a specific area, it's often very hard to find it - especially if you land in trees - so anything you can do to narrow down the search area is important. An AOPA safety video (I forget which one) mentioned an incident where a helicopter went down less than a mile from a runway threshold at a class B airport and it still took 24 hours to locate it (the details may be different, but you get the idea).
The biggest thing that comes to mind is that even if somebody saw your aircraft crash (or land off airport), they may not know to respond, or who to contact. A flight plan has aircraft and pilot info attached, including emergency contact.
I'm not sure that the assumption that somebody will see the crash is quite right, but it's certainly more likely than in the backcountry!
Another reason or benefit to filIng a VFR flight plan when flying over a populated area is if IMC is remotely possible. Depending on where you live and when you fly, possible might develop to probable, then advance to definitely, rather quickly. If you have a VFR flight plan filed, you will already be in the FSS system. It is then easier to convert that VFR flight plan into a pop-up IFR flight plan in order to affect an instrument approach to landing.
Think of situations when IMC moves in to your area underneath you’re flight altitude. Or, the times when you are flying VFR-over-the-top expecting it to be VMC at your destination.