One of the purposes of a cover letter is to remind the agent you're sending stuff to that she met you or requested pages.
I received manuscript pages in the mail today with a green sticky that said "here it is.".
Great. Who are you? I searched for both your name, your email and the book title on my hard drive. Nada.
I looked at my Excel spreadsheet listing all the people I met at the BEA pitch slam. Nada again.
I have no clue who you are, what this novel is, or if it's something I want to read. And frankly, I'm baffled as to why anyone would think this is the way to send stuff.
It may very well be that we did meet, or correspond, and I do want to read this, but right now I'm not reading 20 pages to find out if I remember you.
Be smart: remind me how we met. Even if I emailed you and asked for pages. Even if I met you at BEA. Just say "following up on our conversation on 5/30."