Some heat pumps have a oil trap just before the compressor. why is it there on some but not all?

I'm working with a 5 ton Lennox heat pump that has had 3 compressors put in it and its only 4.5 yrs old. It has a suction line dryer between the acilator and the compressor. This 3rd compressor is locked up. I dont know why . Could it be that someone has alterd the freon pipes so they could add a dryer? thanks