Sure, knives are utilitarian tools that are meant to be used, but they can also be preserved in mint condition as sentimental heirlooms, collectibles, investments or even art objects in some cases. (Speaking of investments, many of my mint antique slipjoints are worth around four or five times what I paid for them, though it took a decade or so for them to get there.) You can only use so many different knives before your accumulation of "tools" becomes hopelessly redundant. Even the most dedicated carpenter needs only so many hammers or saws to get the job done.
Once the tool turns into an obsessively acquisitive hobby--and you probably wouldn't be posting here if that weren't the case--you're eventually going to end up with more redundant "tools" than any rational person needs. At that point, why not squirrel away your favorite or most valuable knives in order to preserve and enjoy their minty-freshness for a while? A collector's taste often changes over time, and a mint knife is always more desirable and valuable when you decide to exchange it in favor of a new favorite.
This subject pops up with fair regularity, and it's ironic that most of the replies consist of variations on
"it's a tool, dude - use it" and
"you can't take it with you," etc., while the majority of knives offered on the BF Exchange are
"NIB; never carried, used or sharpened." 
Depending of the knife in question, preservation and "caretaker obligation" can be an enjoyable part of collecting, too. You don't necessarily have to burn through everything you get your greasy mitts on as quickly as possible.
