I’m focusing on the home, here; being organized in the workplace is a great thing, too.
Time – Figure out about how much time you usually spend looking for items; then multiply that by your usual hourly wage. This is how much you’ll save by being organized. Even if you begin spending more time than you did before on maintaining your space, it should be less than you wasted searching for items (and if it’s not, then systems/methods need adjusting!). Being able to put your hand on items when you need them, instead of having to move things around to get to them, will also save you time.
Paperwork – Organized paperwork allows you to pay your bills on time, saving you money in late fees, penalties, and interest; this is also good for your credit rating. It also allows you to find checks, rebates, and coupons before they expire.
Health – If you can reach all areas in your home for cleaning, you can avoid buildups of dust, mold, and pet hair, and even sometimes infestations (your pantry will be clear enough for you to notice mouse droppings, for instance). This may reduce your risk for various medical problems. You will also avoid injuries by having a clear path for walking, with no items around to trip or slip on, or bump into, either from the clutter itself or your efforts to move around it. Improved health, besides being preferable in itself, will also save you money in medical bills, medicine, and lost work time.
Knowing what you have – If you know what you own, you won’t buy more of the same item by mistake.
Preserving the stuff you use and love – Being organized allows you to keep items in a way that will keep them from getting damaged; for instance, storing papers or fabrics in a dry area and fragile things in a safe space. Disorganization often leads to stuff getting damaged: by being crushed under other things, getting dirty, being water damaged, dropped on the floor, or damaged from disuse (such as batteries in an item leaking acid onto the contacts).
[Added later:] The cost of space! Living in the Metro Boston area, how could I forget? The excess stuff you have costs you money to keep it, whether that means owning/renting a bigger home than you otherwise might, or paying for offsite storage.
Plus, some of the stuff you own but don’t need or love may be able to bring you some income if you sell it.
This is just the list off the top of my head; if anyone has any other ideas, I’d love to hear them! Polite dissent is also welcome.