The net is for finally capturing the subdued fish. Some experienced anglers capture the fish by hand but this can harm the fish if not done with care (a problem for catch and release) and there is the obvious risk of the fish getting away (bye bye dinner).
The net must have a knotless mesh and preferably a soft mesh if fishing catch and release.
The size of the net opening will depend on the size of fish you expect will be a factor.
River fishermen will use either a paddle shaped net or folding net hanging from their side or back (attached by a bungee) to allow them to roam along the river. A net scabbard is useful to keep the net from becoming caught up in the bushes.
Boat fishermen will usually have a long handled or telescopic handled net so they can reach the fish from inside the boat. You don't want to hang the net from your side as it is uncomfortable for sitting. You also won't want to be fumbling around setting up a folding net when you have a big fish on the line.