Here is the FAQ that will best answer what a Reserve is: https://winningcause.zendesk.com/hc/en-us/articles/210500267-What-s-a-Reserve-
But to answer your question of why Reserves are used, well to be honest, we don't recommend using a Reserve unless there is a cost related to the item--in other words, items that weren't donated, items that were purchased at a discount.
However, a Reserve can also be used while also starting the bidding low in order to attract more bidders. I'm guessing the seller was attempting to do this, yet was protecting the donation so that it didn't sell for too little in the scenario where the seller could not reach enough bidders to make bidding competitive.
In the event that you are the leading bidder (not the winning bidder) and the Reserve is unmet, your card will not be charged. Cards are only charged when the Reserve is met AND when your bid is the winning bid.