You can see this for instance when players discuss a free kick, or talk to the referee, or when talking to a colleague on the pitch. I understand the fear of being spied on by someone and communicating information to the opponent. But really? Has this ever been a thing? I mean, no one can pass such quickly passed information to anyone. For example, when two players discuss a free-kick, no one will be able to inform the goalkeeper of anything.
And also, can untrained people really read the lips of other players so easily, especially if it is in another language? Players might suspect a translator off the pitch, perhaps looking with binoculars, could help with the translation, but again, this information, if ever accurate, could be very slow to transmit, and hence become useless.
So, the real question is, why bother? Has such information ever been used against a team? This trend in covering the mouth became widespread, in my experience, around 5 years ago. Thus, for example, in most World Cups ever played, this was not a thing. Why the sudden concern? Does FIFA have anything to say about this?