If I understand it all correctly, it used to be to do with radio failure procedures - and perhaps, from years gone by, the control method used.
The radio failure procedures useed to be different if the clearance included 'request level change en-route' as opposed to 'climb when instructed by radar' - which I guess is equally bleedin' obvious. I suspect that the request level change type clearance technically was a clearance issued in a procedural control environment whereas climb when instructed was for a radar control environment. In my experience many controllers either use the two types of clearance interchangeably of use one type simply in preference to the other. As you can tell, I certainly don't know the detailed differences between the two types of clearance.
The RT fail procedures were changed in 2002 and I think this distinction has now gone. The procedures now reflect what has actually happened for years (certainly in the UK) in that if an aircraft goes radio fail, ATC try and get everything out of its way and watch it closely in case it doesn't do what's expected!
[Edited to say that Tori can't half type fast!]