trying to understand bank's motivation for bid at auction

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I've heard about banks bidding at auctions - would this only be for the sole purpose of raising the bid to try to cover their lien on the property? Otherwise, I don't understand why a bank would send an agent to the auction.

For properties whose mortgages have been sold to secondary mortgage market - the bank who is managing the mortgage payments - they aren't under any obligation to try to get the house sold at a decent price or are they? At which point during the foreclosure process does the bank hand the property over to Freddie Mac, etc.?

Also - I am looking for stats on what percent of mortgages are currently sold off to the secondary mortgage market and am having trouble finding it. I guess it doesn't matter if the foreclosure process is the same up to the auction but I'm trying to get a better feel for what the bank's role/responsibility is up to this point.

Any help/direction would be much appreciated. Thanks!