Being the way we are, we all have this strange compulsion to own something that most others don't have. The rarer the stamp, the more valuable it is. The same could be said of all other collectable items, from cigarette cards and empty beer cans to rickety old furniture which we quaintly call antique.
Right. If there are no surviving copies of a particular stamp in brand-new, unused form, as is often the case with older stamps, then a used one will have to do... and it will become valuable for that reason. Some collectors would rather have a used stamp than no stamp at all, if a used one is the only one they can obtain.
Ahh.. so corrideo's question below was in the context of stamp collectors! Understood - my dad's has an incredible collection too - but I've never seen many new ones in it. Thank you for your answers!
Just a little addition: There have been examples of a stamp being issued but actually selling out to pre-issue orders from philatelists and never getting to any post office (I know of one such example abroad from the Sixties). In those cases it is conceiveable that none are ever used for postage - unless a few of them find their way out and then used ones will approach commanding almost any price (the example is listed used at double the value of unused). There is another complication, in many cases in the early years collectors often kept unused but not so much used ones making used ones now rare - rarity always pushes up values. Finally, to create used (more valuable) ones collectors and/or speculators have been known to later get unused ones "franked" but there is an entire field of specialisation in recognising such "forgeries" (i.e. those that have not actually gone through the postal system) by analysing the franking stamps used (whether they were contemporaries of the stamp), the ink (same type of check) and whether the whole thing is an elaborate forgery altogether (back-room type).
used stamps are valuable to some charities as they can sell them to people who make up packs to sell to collectors in other countries. Guide dogs for the blind does this i believe