Dry goods, and of course, water storage. The 2 liter bottles are essentially waterproof containers that are small enough for daily usage. Anything that can get poured down a funnel, such as rice, coffee, or couscous, can be stored in the bottles.

EDIT: I forgot about the self watering and DIY container/tower garden projects that use 2 liter bottles. For someone with very limited space, it offers an alternative to conventional backyard vegetable gardens.