MG42's are the best infantry mg's in the game, they have decent firepower with the largest range of any Machine Gun combined with the fastest suppression and pinning effect around. Early in the game MG42's are mainly used to suppress enemy infantry while other Tier1 units deal out the damage, the MG is great for suppressing infantry in situations that range from making a heavy assault to defending an important strategic location.
The biggest weakness MG42's have is they can be flanked quite easilly by jeeps or riflemen, it only takes one squad to flank and dispose of an mg so if one of your mg42's is going to be flanked and killed, bite the bullet and retreat it before the enemy squad not only destroy your mg but take the machine gun and use it for themselves. Make sure mg42's aren't flanked in combat by covering them with more mg's or volksgrenadiers and if your mg's do get flanked, make the enemy take much more casualties than you do and set the mg up in a defensive position so that if it dies, volksgrenadiers can recrew it for a very small manpower cost.
MG42's are great at defending strategic points or inside buildings & bunkers as they have a 360' view to shoot at any direction of incoming enemy units. But be warned that mg's destroyed inside a building or bunker can't be remanned and that grenades hitting your mg in a building can be quite lethal, destroying the whole squad in some cases, for bunkers, grenades don't do much damage at all. To dodge a grenade the mg can quickly be evacuated from buildings by pressing the U key, quickly select the mg after and regarrison it or retreat it depending on the situation.
