I want to be cautious in my answer because it would be pure speculation as to "why" for cyclist would use HGH specifically. Like you say if it were for bodybuilding I could give a more clear answer.
Anabolics and androgens would be very useful for recovery from intense training.
Pro cyclist have the advantage when lighter on the bike, but one can use anabolics at a lighter dose (with the right drug selection) and obtain greater recovery ability without much muscle gain. To gain muscularity is like adding ingredients to making bread and is not near as easy as people may think. It takes the right genetics first, but it even with the right genetics it takes the right type of resistance training, nutrition and recovery to add lean muscle mass.
You can give anabolics to a sedentary person and their appearance may not change at all or if their genetics side more with mesomorphic they can become leaner and stronger with very little stimulation. However, most competitive cyclist are ectomorphs by genetics and have a very low ability to add mass. Therefore they can probably use anabolics for the recovery aspect and not worry too much about adding lean mass. By the sure nature of cycling training it is not conducive to adding muscle mass.
Why HGH?
Not exactly sure other than there is a small potential for the same recovery aspects and it is hard to detect with tests if it can be tested at all. I don't want to get to deep on the HGH as to why I think it is inefficient for cyclist not to mention that it impact blood sugar levels in some people forcing them to use insulin as a supplement to HGH.
Many competitors are always seeking to get the edge on their competition so they will use anything that has even the slightest posibility to gaining that edge. If they can get a 5% gain they will try it. The proof is on this very forum where I have answered questions as of late on the "Health and Nutrition" thread from a someone looking at supplements to gain that edge.
Oops!!! I have to cut this short. I have to get ready for a group ride this morning.
Maybe I can pick it up later.