I read somewhere that it is there to be used in 3rd party systems, and that is will also be useful for when webflow allow to change email, in which case this will be the only consistent id.
But looking through the code, it looks like it’s just a hash of the users’ email, so I’m not sure how it will be consistent if the email changes, or why is it better then using the email itself.
Thanks 
Not a lot. If Webflow did allow the email to be changed, then it would no longer be viable as a persistent user identifier.
It was an attempt to provide some form of unique user identifier that isn’t a plain-text user email which is legally PII in most jurisdictions.
In our own internal integrations, we take one of two approaches-
Both approaches work. The alt user ID was a makeshift first attempt but has limited utility.
I see, thank you for the detailed response
Originally I was looking for a way to authenticate a user against an external API for a basic access control mechanism where only certain users are allowed to update certain objects… Do you think this is something that can be done with webflow user accounts, or will we have to also roll out our custom own management?
And also, out of curiosity, what is this advanced RP framework? 
No. Webflow’s is primarily a closed system.
You can do limited input and output from Webflow’s framework through the API but it does not have e.g. the ability to plug in external authentication systems, or ecom transaction system, or external CMS’s…
If you want to auth against an external system, you’ll need a different auth framework.
Sygnal has a proprietary tech framework we call Hyperflow, which adds a lot of capabilities to Webflow-hosted sites. One of the thing it can do is extend User Accounts, provide conditional vis and content gating by access group, partially gated pages, and other advanced stuff.
It’s not a product. Just an infrastructure we customize and integrate for our larger clients who need those features.