You’re right that captchas can be bypassed, but I disagree that they’re useless.
Do you lock your house? Are you aware that most locks can be picked and windows can be smashed?
captchas can be defeated, but that doesn’t mean they’re useless - they increase the level of friction required to automate malicious activity. Maybe not a lot, but along with other measures, it may make it tricky enough to circumvent that it discourages a good percentage of bot spammers. It’s the “Swiss cheese” model of security.
Registration applications stop bots, but it also stops legitimate users. I almost didn’t get onto the fediverse because of registration applications. I filled out applications at lemmy.ml and beehaw.org, and then forgot about it. Two days later, I got reminded of the fediverse, and luckily I found this instance that didn’t require some sort of application to join.