I used them because I found the insoles most manufacturers put in are rubbish. They rarely provide any padding so now I always have a pair in my boots, especially while trekking when you're going to be living in your boots. I buy a new pair for any new boot I get, so I don't forget them if I use a different boot.
I did once get persuaded to try those horrendously expensive inserts that are supposed to correct your foot position, but found them equally horrendous to wear. That could just me the shape of my feet though.
Incidentally, I used to work with an ex-army Gent (to give you an idea of timescale he was in Kosovo), and he told me that a lot of the military chuck away their army issue boots, go to Altberg in Richmond for made to measure boots and put in a Sorbothane insert.
S.
