Consolidate the things that float around in the bottom of your bag. Great for people who aren't fans of pockets, but still like to keep their stuff organized.
Carry pens, pencils, makeup, toothpaste and toothbrush, etc. Use it as a clutch when you go out for lunch.
When your sunscreen tube bursts, the Velcro flap keeps the goo contained, and the liner makes it easy to clean.
Ortlieb Pannier Carry System
Ortlieb Saddle Bag Mounting Straps for Spring Saddle
Chrome Laptop Sleeve