Agree with the above. There is also the need to get the LL departures to the east of the airway to allow the Midlands controller to descend the Birmingham and Coventry inbounds to the west of the airway towards DTY.
If an aircraft is left on the WOBUN SID in a busy/complex traffic situation, the options for climbing, positioning and getting it out of the way are drastically reduced as it ends up stuck underneath SS/GW/LC/KB inbounds and then after that the BB/BE/NX inbounds. Positioning the WOBUNs out to the east gives the controller much more space to get you climbing up and out of the road of everyone else.
As coracle says, it's much better explained if you're sitting there watching us work. Visits from pilots are very welcome, not only so they can see what we do but for us to fire 101 questions at them!