Please can someone explain the purpose and benefits of external wastegates?

 

If you call waking the dead a benifit, there's one.

In effect when the wastegate is open you split the exhaust gas then re-join it (at different velocies). By not having the by-passed exhaust gasses re-joining at the down pipe, you can, technically have a smoother exhaust flow. Both because you no longer have a gas joining the exhaust at a different velocity, and you can have a less interupted downpipe design.

I don't know if these have any real world gains, but technically it should give the advantages of both single dump downpipes and twin dump downpipes if done right.

Please excuse me if i'm speaking out my A*** as this seems to happen regularly these days!

 

You can also flow more air through an external wastegate, and boost control can be superior.

 

Mine certainly doesn't wake the dead. Just because it's external to the turbo doesn't mean that is can't rejoin the main exhaust system further down the line. Not that I'm meaning to imply that anyone is speaking from a non facial orifice.

As JB says, Boost control is very good with an external wastegate, and they have sufficient capacity to control very big turbos.