My mate told me about this, I mentioned it the other day.
He's been doing some fitness thing that Liverpool coaches lay on for free (he had a brain tumour last year so was contacted about it), & he had them doing it in the match.
The idea behind it is to keep the line, obv, when you're defending set pieces, but also if you're in a wall for a free kick you tend to all push against each other which moves the wall from side to side & allows the opposition to stand in the wall or next to it do the same.
It also stops opposition players making late runs into the six yard box at corners cos they have no gap to run through.
The coach said technically it could be classed as obstruction if a player tries to run through their hands, but because it's in the area there's no ref that will give a penalty on the back of it!