Reserves are set by sellers primarily so they can get people to start bidding on an item to spark more interest. More people bid on items with bids, than those without.
For example, I have an item I'm willing to sell for $50. If I start it at $50, people tend not to place a bid. But, if I start it out at $10 and get the ball rolling, it might very well meet the reserve and surpase it when people start the bidding war.
Either way, don't worry about it. AS gives you the chance to bid what you're willing to pay. If you saw an auction with my example, it wouldn't really matter what the minimum or reserve is. The only advantage you'd have of knowing the minimum is $50 would be you wouldn't place a bid if you thought it was only worth $40. If the seller has a reserve and low initial bid, bid what you want. If it doesn't meet it, keep looking for another item.
There are those that use the reserve to make sure they get the maximum $$$ out of the auction. I make it a policy to only buy directly off eBay and not make personal offers. That way, I have some amount of buyer protection.
Dave