Sea To Summit says:

The waterproof Sea to Summit eVAC Dry Sack allows air to compress out without letting water in, keeping the contents dry while saving space in your pack.

We use waterproof, air permeable eVent® fabric on the base for valve-free air expulsion, allowing compact packing. Instead of straps for compression, just roll the sack down and air is pushed out of the base. Then close the buckle and you’ve just saved space. These sacks are oval-shaped, rather than round so they better fit the internal contours of a backpack.

It’s an excellent option for storing and compressing clothes and sleeping bags without the use of straps.