Not sure if having an option available requires logic to justify it, or whether eBay has even considered whether the option should appear if the seller has "make an offer" already available.
In any case: some buyers are hesitant to make an offer. Perhaps such buyers have had bad experiences with previous offers that were rejected by sellers that did not want to receive any offers (where eBay added that option against the seller's wishes); perhaps such a buyer was blocked from making a purchase after an offer was considered "insulting".
A seller that accepts offers may still want the option to make an offer to a buyer. Perhaps a buyer, receiving such an offer, might consider that others might have received the same offer and might buy the item if the original buyer does not, and thus be prompted to act on the offer.
