They have normally been good for two things. One, they allow you to get the latest release of the retail OS you bought for $20 with one coupon. So say you bought Tiger and it came as 10.4.3. In the future, you'd like to have a retail 10.4.11 disk. Use a coupon and $20 to get a new retail Tiger disk. It will have the latest version of the same level OS plus all updates at the time of release included. Install from the new disk and you won't have to chase after the Combo updater, latest version of QuickTime, security updates and such. At least if they haven't been superseded after the disk was made.

Second, they're good for (usually unadvertised) specials. One person here a while back asked what the coupons were good for when he happened to be in an Apple store. The clerk told him they were currently good for X amount off a few things in the store. He had gone in to buy speakers. The coupon made the price on better speakers less than the ones he was looking at originally.