It benefits a seller in that it MAY help sell the house. Example-- you have your downpayment money but lack closing money. IF the house will appraise high enough the seller consents to include the amount needed (usually up to 6% of selling price) in the selling price thus giving you the needed funds by "paying " them for you. You still wind up paying for them indirectly in the price of the house.
Aside from closing costs a seller can give you a "concession" to replace/repair things that they can otherwise deal with if the house does not sell. Example--carpeting/flooring, a septic upgrade, appliances. It is an incentive to get you to buy it knowing you will get funds to do it on your own.
A seller's concession benefits the seller because it assists the buyer in the purchase when they do not have the funds for closing costs.
Let's say the house is selling for $100k. The buyer will typically need 4-5% for closing costs in NY. So, he buys the house from you for $140k, but you pay the $40k in closing costs. Tax wise, it is a wash. You sold for $40 higher and paid closing costs of $40 which is 100% tax deductible.
Caveat: The house must appraise for the additional amount and the buyer must have great credit!
In most cases, the seller is mainly concerned with what they are netting..meaning how much money they are actually walking away with. A Seller's Concession is a tool to help a potential buyer qualify to purchase. Assuming the home appraises out there is very little impact on the seller