Advantage of Allowances to Seller?

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I'm currently working on purchasing a duplex. I have made an offer, and the seller has countered. The duplex will probably be in need of a new roof in about 5 years. The seller has mentioned a couple of times, that if there are things I think need fixed, that he would be willing to offer allowances for them. What is the advantage, to the seller, of offering an allowance over just taking that same amount off the selling price. For example: why would he offer a $5000 allowance towards roof repair instead of just taking $5000 off the selling price? Is there a tax advantage or something?

A new roof certainly would be nice, but if it still has 5 years of life left in it, then I would prefer to get 5 years out of it, instead of getting it replaced now.