You might want to have internal version numbers of your web application to be able to do better internal planning. However, these numbers are of little use for the user.

The usual purpose of version numbers is to tell the user if their application is up-to-date or not. When this is not within their control, it can at least be useful for support personnel to verify that they have the latest version. But when you have a software-as-a-service where all users share the same version, there is little reason to communicate the version number to them.

An exception could be when you aren't the only one who offers the service. When you only licensed the software or are licensing it yourself so others also offer the same software as a service, it could be useful to communicate the version number to the end-user so they can compare which provider offers which version and also be aware of the version used by their provider in case there are relevant differences in usage.