Maybe the big difference here is getting people to book slots?
Booking slots helps emphasize that this us a one-on-one opportunity for a conversation - not a situation where you might feel awkward that there are several others on the chat who already know each other.
It also sets up a small obligation that you'll actually show up, since if you reserved a slot it's rude to cancel at short notice.
In the educational setting, this is a massive effect. If I offer students feedback slots they can book, generally a majority will book a slot, and around 90% of those will turn up. If I just offer times they can drop in for feedback, I get maybe 5% dropping in.
(The course is behavioural economics and this is a kind of interesting behavioural phenomenon).
Booking slots helps emphasize that this us a one-on-one opportunity for a conversation - not a situation where you might feel awkward that there are several others on the chat who already know each other.
It also sets up a small obligation that you'll actually show up, since if you reserved a slot it's rude to cancel at short notice.