WEC different classes in the same race - why??

[Mark34bn]

I'm just watching the Spa 6hrs and was wondering why they have the different classes competing in the same race. The speed differential is huge between the Porsches / Audis and the GT cars.
I love the racing but I don't know of any other series that has cars with such different performance.

Anyone know why it is like this?

[Mark34bn]

I can't wait for Le Mans either. Eurosport had full 24hr coverage last year.