Well, I should mention that not everyone uses an Alpha reader. And it depends a bit on your process. So let me back up a bit and tell you mine.
For me, the beta readers get a book that is pretty much done. Yes, they find things that need changing, but they are minor and require a little tweak here and a little tweak there. They also confirm or deny things I’m not sure of like did I wait too long to introduce this character, was this part of the book interesting or boring, things like that.
Again, for me, the alpha reader is my wife and she’s my alpha because (a) she’s the most intelligent person I know (b) we’ve been together for so long that I can take criticism from her without taking too much offense and © she’ll be honest and not pull punches and be willing to stand up for something she thinks is seriously wrong with the book and (d) has a great eye for consistency checks, plot holes, character motivation, and pacing.
Robin never reads any of my works in progress. She only gets the book once I feel it is done and solid. But in almost all cases she has changes that I didn’t even see. Some major, some minor, but almost all of them need me to address them in one way or another. She’s made me change how a person dies, if they die, or if a character is even needed in the first place. She’s shown me when my characters do something against their nature. She looks for problems with pacing, and information that comes at the wrong time. In short, she’s a developmental editor, and she’s great at what she does. I’d pit her against any editor at any of the big houses, and it’s because of her work that my publishers rarely have any substantive feedback. Between my alpha and beta readers, the publishers get a book that really only needs copy editing, and not much more.
Again, I’m sure different people use (or don’t have) alphas at all, but in my process this is what my alpha does.