I've purchased a fair number of early access games from indie developers.

For me the benefit is that it's often cheaper during EA, so I get it at a discount, and it already feels like a complete game worth the price I'm paying. I know they are actively working on adding more to it, and having more things added to the game for me to explore extends its lifespan for me. So I get more enjoyment out of it than I would waiting for 1.0, at a cheaper price.

For small developers it gives them the funds to continue development, and feedback that helps with game balance.