PKCS#1: Purpose of `Padding1` in EMSA-PSS encoding method
I conjecture that this salt is essentially a version number for future extensions, tasked with ensuring that signature per another version using a different salt is not a valid signature for the version described.
I can imagine two other roles:

It makes it even less likely that an RSASSA-PSS signature comes to match an RSA-PSS signature of something pre-existing starting with a hash (but there's also the 0xBC for right padding of EM playing that role). Recall that RSA-PSS was introducted by Mihir Bellare and Phillip Rogaway's The Exact Security of Digital Signatures - How to Sign with RSA and Rabin, in proceedings of EuroCrypt 1996; and it's reduction to practice RSASSA-PSS is a slight variation, as explained in notes of section 9.1 of PKCS#1v2.2.
It makes it less untrue that the hash function / random oracle producing $M'$ is independent from the one producing $\text{mHash}$ and the one used inside MFG1, which arguably is a silent assumption in the security argument.