It is definitely an ease of carry thing. I have a bradford guardian 3 that I really like especially if I know I am going to be doing something fairly messy...no issue if I am out grilling to use a knife like that to cut up some raw meat it is far easier to just rinse and move along without worrying about the issues involved with getting gunk in the pivot or on the lock face of my folder.
I have heard others also do this for the purpose of keeping one of the knives super sharp and use the other for all the utility things. I myself don't really do this but I can't dispute the logic. Need to break down cardboard? Just use the small fixed knife with a working edge rather than taking a freshly polished m390 folder edge to it.
Others have said (and I don't subscribe to this im just the messenger) that they can use a small fixed blade as a scraper, pry bar, screw driver, etc. Some "hard use" things that dont really involve batoning the spine but likewise you wouldn't want to use a thinner, fragile, possibly more expensive folding knife for. Again, not my argument but one that I have heard for the smaller fixed blade and larger folder.