Obviously you have never had a VPN,.
I have for work.
that is entire idea of it, it changes at least every 20 minutes..
No its not. The two main reasons/uses for VPN's are 1) to encrypt endpoints as @Logged-Off says, and 2) to provide IP in different geo locations (or different provider in same geo location) to bypass geo blocking.
While IP rotation may be an option, its far from the "entire idea of it".
@vk3col Mac using Checkpoint VPN and Windows 10 PC OpenVPN both using MS Outlook for Email and no issues.
I believe it is a configuration issue this link may help