How much history do we need?

Oh, dear, sometimes I find a good quote online and do a quick screen-capture, thinking I'll use it right away and REMEMBER where I found it.  Well, duh.  My memory is iffy in the best circumstances, but if something sits in my files for weeks, it's a total provenance toss-up.
This, for instance, I marked "Vol 1, Creameries"....(probably from that History of Farming in Minnesota book Larry found).  Anyway, it explains a lot:

Remember the cheese factory east of Rice that dad used to point out?  It was at the top of a rise, and there were two very tall pines in the yard.  There's a house there now, and no pines. It's too bad that so much history just dissolved, ya know?  It was there in the background of mom and dad's lives, a landmark at least and a place to buy cheese at best...but whoever built it saw it as a needed industry and a good way to make a living in the middle of all those farms.  
On the other hand, I follow something called The History Blog online, and it seems like there are thousands of "incredibly important" ruins all over the world.  It makes me wonder if EVERY SINGLE ONE really needs preserving?   How much more do we honestly need to know about medieval London or Rome...or, ya know, rural Rice?  
No answers here...I remember the Cheese Factory Corner and just hope that someone else does, too.  That'd be enough, I suppose...