If you just jump in forwards and the tank straps are loose, when the tanks hit the water they could hit the back of your head - not a nice thing to happen, as I know from experience!! There's also the risk of your mask being dislodged when you hit the water unless you're holding on to it tightly. Once in the water, the boat will be behind you - somewhere - and since you can't see it you could collide with it as you surface. You also have to turn round to see the boatman to let him know whether all is well, or you need assistance.
Falling in backward, your tanks hit the water first and this helps break your fall. Your mask isn't subject to a strong rush of water, and so stays in place. You can see the boat even before you surface, and avoid contact with it. You can also easily see the boatman, and signal straight away that all is well, (or not!!)