assuming you are talking about an auction house (that takes a %) rather than an online auction site (that charges per listing)

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The reserve gives the auctioneer a guide. he won't sell your thing for less than you will accept just to get his percent

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For online.. well. they get their listing fee (and if for example the interested parties have to use the auction houses online payment service, they are guaranteed a transaction fee also)

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But starting bidding at below reserve allows action to happen. If you ever go to an auction it's tempting to get all excited over an item and stick your hand up. Once that feeling passes you just chill on an item you like, allow the auctioneer to keep lowering the price then get involved once the action starts

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For online, maybe there is 50 watchers but everyone is looking for a bargain cost of living cris and all that, by allowing anyone to bid 50p with 10minutes to go when the reserve is £50, all of a sudden a whole bunch of preset bids can be triggered and just maybe the price goes to reserve or above

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By not showing the reserve then (as others have said) this can help to start action and drive up the price, which is exactly what the auctioneer wants (for his % to improve in the example of bricks and mortar) or you do actually get a sale and then the auctioneer gets their transaction fee also (for example ebay) and is best for you (so you get more £ from the sale)