It very often that after purchasing 3D printer (especially in Anet A8, A6, A2) motherboard is getting all load to heat up Printing Hotbed. That creates fire hazard problems as connectors are not suitable for that high loads.
Hence that issue MOSFET Module is solving existing problems by taking the load off the motherboard.
It also solving print quality issues as motherboard is not heavily overloaded with power consumption of heating printbed.
It is used not for hotbed only. It is very common that it is used for extruder's heating element too.
MAinly it is to take the load off the motherboard and make motherboard more stable and power interference free.