Ankle mobility is important for swimming well and with the least drag. Swimming with your feet not pointed behind you means you have two feet that are creating drag as you swim.
Sometimes it’s a motor control problem. You tell the swimmer to keep the toes pointed behind them and they imprint it, and just keep doing it. They don’t have any inherent mobility problem but rather they just weren’t keeping their attention on it, and now they are and they can do it.
For others, pointing the toes behind them as swim can be hard if not impossible. This is because they have restrictions in their muscles and soft tissues around the ankle that hold them in their normal bent position and getting out of that position can take a lot of effort if the soft tissues aren’t addressed. If you ask these swimmers to point their toes, they will either 1) point for a few strokes but when they get tired they revert back to not pointing, or 2) they will cramp their calves which will be overworked to keep their toes pointed.
Watch this video for some ideas on increasing your ankle mobility: