Maybe someone else can come in and do the organic chemistry explanation as I've been up for quite a bit too long to handle it correctly, but it's VERY easy to see the difference for yourself with 1 print.
Coat your paper, dry it with a hair dryer to bone dry, rip the paper in half.
Take one half and expose it right away with your negative.
Let the remaining half sit in the dark for 1/2 an hour so it has time to come back up to ambient humidity in the room (I live in Fort Worth, Texas, and our ambient humidity is rather high during this time of year, but I just invested in a humidifier to humidify my papers before printing) before printing. If you're using a vacuum frame, make sure there is something between the paper and the vacuum frame (mylar and the plasticky foam core work well for this) to help keep the moisture from leaking out.
The first print will be very anemic and have a look that I like to describe as "cracking" blacks, like they just kind of "break off". This is a rather poor description, but for those that have seen it they have used similar language to describe the blacks. The second print will be much closer to the lush, deep black that is attainable with pt/pd printing.
Now there are LOTS of other variables involved in this, but the *biggest* problem to getting deep blacks I have seen new practitioners make is not printing with a high enough moisture content in the paper during the exposure. Once we do this little test they are amazed by the difference.