Isn't Prezi Desktop completely offline?

Dear Sirs,
I think I am missing something, regarding accessibility to Prezi Desktop offline, with the Pro Plus licence, which I have:
I’d like to login in Prezi Desktop while offline, but I don’t know how, because when I write email and password in the login page, the system needs to be online to let me in…
Can you please explain ?

Thanks,
Davide

Thanks for the explanation, Vanda.

Let me ask you one more question with this example:

let’s assume I need to do a presentation 2 hours away from the last spot (my home) where I have internet connection.  
In such case I should sign in at home, leave the computer switched on, and use Prezi Desktop upon destination…
What if my computer switches off in the meantime?  When I’ll switch it on again, I’ll need an internet connection to login… but in this example I sinply don’t have it…, so I cannot use Prezi Desktop…

Did I get it correct?   If so, I believe Prezi Desktop is not “totally” internet-line free…

Please advise and thank you,
Davide

This is clear to me now, Vanda.
Thanks for the explanation!

Davide