Going to writing conferences will increase you chances that a literary agent will read your manuscript, compared to the chances if you simply submit over the transom. It will not make it a more publishable manuscript.
Why do agents go to conferences (a significant cost in time, if nothing else)? Because they make their living on 15% of author's royalties and they need to find professional writers with salable stories to represent. The problem for agents is that 110% of the adult population of the planet now thinks they are a novelist and thus agents and publishers are constantly bombarded with manuscripts that are complete drek.
The chances of finding professional writers with salable stories by reading these piles of drek is pretty low, so agents look for other ways to find prospective clients. If a writer is serious enough about their craft to go take the time and pay the fee to attend a conference and the fee for a pitch session, there is a much better chance that they are serious enough about the craft to commit their resources to it, that they acknowledge they have something to learn and are willing to take the time to study the craft, and that they have taken the time to figure out how the publishing business actually works. Thus there is a much better chance of an agent finding a professional writer with salable stories at a conference than by sitting in the office reading the drek in the slush pile.
Do writers find agents at conferences? Actually, that's the wrong question. The right question is, do agents find writers are conferences? Yes, or they would not be there. And in fact I have seen it happen. At least twice I have seen an agent at a first page session (in which a moderator reads the first pages of ms submitted by attendees to a panel of agents and editors to get their reactions) demand "Who wrote this?" and insist that the author identify themselves and meet them immediately after the session. I can't say for sure if those meetings led to representation and publication, but, again, why else would the agents be there?
None of this helps, of course, if your manuscript is drek. Another function of conferences is that they can provide you the chance to pay for a professional critique from someone in the business that will tell you if your MS is drek or not. And if it is drek, the conference sessions may help you learn how to make it better. Finally, they are a good place to learn the professional jargon of the publishing business, which is important if you want to appear to an agent as a professional writer. (I mention this because all novels are fiction. If you say "fiction novel" to anyone in the trade, it will immediately mark you out as an amateur, and thus greatly reduce the chances that they will read your MS.)
