Why are the Marines in USSOCOM?

why is do the Marines have a unit in SOCOM? They already have Army SF and Rangers for ground combat. SEALS's and SWCC have the water covered and the 160th SOAR and Air Force spec ops have got the air covered. PJ's got the rescue missions, CCT's set up your airfield and spec ops weather for battlefield conditions. With all that said why do you the USMC is in SOCOM? I think it is just for the money but im not 100% on that. what do you think?