Markets are more efficient than you give them credit for. If there was no value-add it wouldn't happen. The value-add is that the scalpers take the risk and hassle off of the artist/venue. They can instantly sell out and then get back to worrying about the actual performance. Selling the tickets at their fair market value initially sounds good in theory, but then you have to spend weeks and months trying to get them sold, which is not a core competency a musical artist really wants to have. It is advantageous to underprice and know that you are sold out upfront and let someone else deal with the slog.
It's much the same as why wholesalers sell to Walmart for pennies on the dollar instead of trying to capture the retail market themselves. Selling direct to retail seems like a good idea... until you have to do it and realize you'd rather get back to what it is you're actually good at producing.