Because being organized means you are taking care of your things. When items are cared for it helps them last longer which reduces having to buy replacement items for those that have been lost or ruined.
Taking care of our items increases the chance that when we finally decide to donate an item it will be reused and brought home by someone else. Case in point: I recently donated an original Xbox to our local thrift store. We took good care of the item and when it was donated everything worked on it, it was clean and I even had the original box and manuals for it along with about 20 games in their cases. After I donated it, the store managers put it on the shelf and it sold in one week for $80. Great income to the charity the thrift store supports, great tax deduction for me, and great pleasure to the person who bought it. Do you suppose the same would have been true had I brought in a dusty old Xbox with no working controllers, missing cables, and scratched up games?
For little kids the simple act of sorting toys is educational. For you time spent assisting your little ones with organizing and cleaning now means they will be able to be more independent with clean up tasks in the future. Plus, kids toys have tons of pieces and by keeping them together they will play with them more frequently.