So, TLS is just a point-to-point encryption protocol, it doesn't prevent anybody of the parties involved from having access to the content. Once the email is encrypted with PGP, Proton loses permanently access to this content.

So this is pretty much what happens with a Gmail <-> Outlook and a Gmail <-> Proton email.

Gmail to outlook:

A writes the email in their editor <- TLS -> Google servers <-TLS-> outlook servers <-TLS-> B reads the email. While every communication is encrypted with TLS, every server has access to its content. Every time B accesses the email from outlook servers (I.e., their inbox), the data is transferred with TLS, but outlook is the "other end of the tunnel", so it has access to this content.

Gmail to Proton:

A writes the email in their editor <- TLS -> Google servers <-TLS-> Proton servers -> encrypt original message with B's public key and discard original -> send to B inbox -> Proton client decrypts email -> B accesses it.

So yes, it is

about making sure your data on the servers stays safe even if someone gains access

As long as you consider the email provider part of those potential "someone".

The way I would say it essentially is that PGP encryption (even in cases where the original messages was not using it) still gives you the confidentiality property of PGP, even without the integrity and non-repudiation properties (which are not possible to guarantee with respect of the original message of course). In other words, the biggest difference is that the email provider doesn't have access to your stuff.