LeapFrog sell a variety of cartridges for some of their products, and these cartridges can be quite pricy, so being able to reuse them with another product or tablet can be useful.
The LeapPad Ultimate does come with a cartridge slot, and most cartridge games should work with it, as there is a lot of cross compatibility with the various tablets, with the LeapFrog Epic being the main one that has problems, as it lacks a cartridge slot.
Content available on cartridges also tends to be available to buy in a downloadable version from the LeapFrog App Center.