I'm going to be very cautious talking about this and only stick to information that is publicly available, which can be found here. Anyone who plays Honkai Star Rail can apply, so it's not like the program itself is a secret. What I do want to talk about are my general feelings about being a beta tester and who may or may not want to apply.
Sometime last year, I received an email about the beta program and applied out of curiosity. I wanted to see how it all worked and yes, I did want to see upcoming content early. I eventually got accepted and I've participated in many tests since. I've personally been enjoying it, but whether or not it's for you depends on the answer to one question: Do you love playing Honkai Star Rail? Like REALLY love it?
I say this as someone who tries to zero turn clear Memory of Chaos, get a perfect score in Pure Fiction, and do the highest levels of Simulated/Divergent Universe, even though there's no material rewards for it. I really enjoy the mechanics of the game, finding the right team compositions and strategies to clear the hardest content as effectively as possible. If you're that kind of sicko, then you would probably like beta testing as much as I have. It scratches that itch.
I've also found it fascinating seeing how the game changes over the course of testing and the final version being released. In fact, I've seen feedback I've given implemented later on. I understand that would only happen if a lot of other people gave that feedback and it's not just me, but still, it's cool to feel like you're part of the process.
If there's one big challenge, it's time. As you might imagine seeing Star Rail's update schedule, things move quickly and need to be done within certain time periods. Combine the time you'll spend on testing with the time it takes to keep up with your real account, and you will be playing a LOT of Honkai Star Rail. Once again, you have to really love playing this game.
Finally, it is cool to get an early look at new content in an official capacity. Again, being very cautious, I'm avoiding details on what exactly you get to experience. But if seeing new content early is all you're interested in, you're probably better off spending your time on something else. However, if you do love playing Star Rail and want to play more, come join the rest of us sickos.