To what extent do literary agents generally get involved in the creative side of a writer's world? That is to say, I understand that the agents are looking for specific materials (as you've said). But, once a writer delivers that, does the agent simply sign the person, accept the manuscript as written, and proceed to hawk it to publishers? Or, does he or she also get involved in the writer's work and suggest creative directions and so forth? (This is assuming the writing is great to begin with, of course. I realize that an agent is not an editor.
Keep in mind, I can only speak to what we do at this agency, so other agents may respond differently. When we sign a client, almost all of the time her work is ready to send out to publishers. However, if minor tweaking is needed, we help our clients with that by offering suggestions and directions. As stated in your comment, we do not do complete edits on a writer’s work before we sign him nor after. This is not our job. Ours is to find a suitable publisher, to negotiate a fair and equitable contract, to advise, and to manage our client’s future career.
There have been cases recently where novels were very close to what was needed to fill an editor’s request and thus I tried to help those writers make their works ready. Unfortunately, most of the time, this has not worked well for me. Either the writer was not at level high enough to achieve a revision and rewrite or took my suggestions and went elsewhere. Consequently, and because of this, I am not as quick to work with someone other than my clients.