This dry bag comes with a valve and compression straps. I understand the purpose of the compression bag, but what is the purpose of a valve?
Letting the air out while you're closing/compressing the bag, looks like.
It's probably more of a risk than a benefit in general, but could be worth considering if you need to pack tight around hard objects or edges, like having gear for a multi day trip inside a kayak.
Alternatively, you could use it as a water bag with tap, but it does not look like that's the intended function.
A further reason is to use the space in the bag for buoyancy and to squeeze the bag into a space securely, by adding air to the bag after it's in place. This generally means a valve on a tube.