>don't you think it makes a big difference to do it IRL rather than on the internet?
Yeah, I think it would make a difference, but only if there was also a real conversation with the person you were confessing to.
For example, if it were totally anonymous and you just sat down, said your mistakes or whatever, and then left, then that would be very similar to an online confession. What would make an irl confession more meaningful would be looking at another person, and hearing that person's reaction or sympathy, or guidance. So I think the value of confession-in the absence of religion- isn't just saying something out loud, or typing it on a keyboard and then leaving. The value would be in having an honest exchange with another person, where you were free to discuss things you wouldn't want to discuss with the people in your daily life. Sort of like professional counseling, but without names.
Of course this type of value can also be created online, so long as the direct and honest exchange were a part of it, but meeting face-to-face will always have more impact imo.
To answer OP's question: The service would probably help some people, but I wouldn't use it myself.