Setting an automatic acceptable Best Offer means the customer is billed as soon as they meet (or surpass) that price.
So they can also pay immediately.
If they have to wait for you to accept the BO they may wander off and forget about eBay for days. This is also a problem with more traditional Auction listings.
You can also set an automatic refusal for BOs that are too low and the customer gets a notice from eBay which must be polite and encouraging enough that they will continue to "bid" until they hit the sweet spot.