Pawns are actually strongest when they are in a horizontal line, ie, they controll more squares, but usually it is dificult to do this without making them succeptable to attack. Therefore if you have them in a "chain" they are harder to attack, but also controll less squares, only one colour of square... if you want to hinder piece movement you have to be aware of where the enemy pieces can move. For instance, if you have a chain on white squares then a dark bishop can easily infiltrate that structure. So, its a bit more complicated than just saying to put them in a chain because they are harder to attack, you have to have a reason and a plan. One other thing is that a chain is more of an anchor to support your pieces. The squares these pawns controll are called outposts.