Most instant messaging apps market themselves on just how much fun they are, with more stickers, emojis etc than everyone else.
Caliber ignores all this by going for a different audience entirely: it's messaging for your business contacts.
The key difference is probably Caliber's LinkedIn integration. The app starts by importing contacts from your LinkedIn account, rather than your phone, and it keeps them synced after that.
This means contact profiles have the same business-oriented flavour, with details on their skills and the companies where they've worked.
You can also try to connect with new professionals, as with LinkedIn, although here that's even less "instant". The system won't hassle you with real-time notifications of any requests, instead sending you weekly summaries, and automatically removing anything you ignore for two weeks in succession.
A straightforward messaging app, maybe handy for LinkedIn users who need a more business-oriented service without the usual distractions.