Dumb newbie questions

I’m not that new to weblogs, I’ve had one for a little more than two years now (MT, soon to be switched over to WP). I’ve also started a new weblog that I’ll be announcing here in a week or so. I really like WP a lot now that I’m used to it. I was just wondering about some of the options. I never really sought to publicize my personal weblog much, but with this new one I’d like to get the word out.
For instance, what is the advantage of custom permalinks? I apprehend that it is an advantage for Google listing?
Another question: the “Excerpt” field. What is the advantage of filling that out, if any? Is it used for the RSS feed? And if it is not filled out, is there a default?
Thanks for your patience. I’ve been able to install several hacks and plugins through these very helpful forums. When it became inevitable to me that it was time to move on from MT, I chose WordPress because of the active and helpful developer community. I hope that in time I become sufficiently familiar with the product to help out with writing documentation and such.