Is there some reason a scalper can buy tickets before you? I assume they are camping out to grab the tickets as soon as they are on sale. No reason you couldn't do the same and score a few.
Now, if you don't want to sit in front of your computer at 5am to buy tickets before they sell out, then the scalper is offering to do that for you for a price.
Yes because scalpers buy as many tickets at one time as they can, lowering your chances of getting tickets even if you are up at 5am trying to buy them. “Brokers snatch up at least 50%, if not more, of these tickets,” says Josh Baron from: https://www.buzzfeed.com/perpetua/6-reasons-why-its-so-hard-...
Scalpers provide this value from buying up many more tickets than an individual would, and individuals who cannot go will still sell their tickets, which is usually for a more reasonable price since they want to get rid of them.