There is a possibility that aircraft on SIDs/STARs could come into conflict with each other or other aircraft within the TMA. Two aircraft that are locked on suitable headings should never come into confliction - As simple as that.
You depart Heathrow for example, on a DVR departure. If you don't want to get higher than 6000ft, then by all means, remain on the SID. If you want to get higher, which I'm sure you would like, expect to be turned off the SID (The DVR SID tracks underneath the BIG stack - If that's full, you wont be going any higher)
The airspace in the UK, the London TMA especially, is incredibly congested. You should try to organise a visit to the UK area units then you can see first hand, the problems that can be experienced. I am sure you would get a lot out of a visit
