To answer this, we need to think about how beaches work. Generally speaking, the water over a beach is not all at the same depth; it is shallow near the shore, and deeper further off. It is considerably easier for heavily laden soldiers to walk through shallow water than it is for them to swim through deep water. As such, your landing craft needs to put them into the shallow water, not the deep water. It is easiest to do this if the troops disembark over the bow of the boat (especially if there's a ramp), rather than over the sides or stern, which will put them into water deep enough for the craft to stay somewhat afloat. Disembarking from a forward ramp also makes things easier for offloading after the beach has been secured; it's a lot more efficient to build just one type of landing craft for both assault and sustainment operations, rather than building one craft for each.