In this case I'd wager two things. As a kid I had family who worked a receiving center for Goodwill. Fairly affluent part of our town near the beach. I remember two distinct things being odd to me then. The items people would bring would sometimes be questionable as to how they'd be useful to the needy, either from wear or function. The other part was most people wanted and received a receipt for their donation. Cue Mitch Hedburg receipt for a donut routine. I was told then when I asked this was an approximate value of their donation and it was used for tax purposes. So one is probably tax write offs.
Throwing things away costs money. When my wife and I moved recently we cleaned house. A second trash can was around 150 a year with limited volume. Trips to the landfill are charged by weight differential. Charity donation is free with the added bonus of someone coming to pick it up if the donation is big enough. We both commented at the time that if we were a little less moral we could easily pack the rubbish in with the donations and save a ton of money. So second is probably convenience with some working the system added in.