It's said that half of an adventure's successes are determined beforehand in town. Actually, nobody's ever said that, but I figured it would lend some credence to my words if I make it sound like an established idea. Really though, asides from preparing spells (which only some of us get to do,) before every adventure it's wise to properly equip oneself. How can you be exactly sure what you need, though? Rope and torches are safe bets, but will you need a mirror? What about a shovel, or a crowbar, or chalk? How much easier would things be if you had antitoxin or holy water? Will there be a water hazard? An airless room? A chasm too wide to cross...?
Forewarned is forearmed, but most of the time you'll ultimately be guessing at what obstacles you'll encounter, or simply settling for the most standard useful items and hoping ingenuity will take care of the rest. After all, it's not like you can afford to haul around every single mundane tool and semi-magic item in existence 'just in case.' Wouldn't it make things so much easier if you could, say, choose which items you brought after the fact? I'm not advocating for lying about what's written on your equipment list, but rather relying on a single item that can replace a whole suite of mundane tools on a whim!