You only need a memory manager for pointer protection. Intel states their intent for SGX on their website[1]. Some of the more important goals are:

    1. Allow application developers to protect sensitive data
       from unauthorized access or modification by rogue software
       running at higher privilege levels.

    ...

    5. Enable the development of trusted applications [...]

    6, Enable software vendors to deliver trusted applications and updates [...]

    ...

    8. Enable applications to define secure regions of code and data that
       maintain confidentiality even when an attacker has physical control
       of the platform and can conduct direct attacks on memory.

This isn't about buffer overflows. You don't need to create a system where software vendors can create regions of memory that are protected against a logic analyser or cold boot attack to fix C style pointer bugs. You do need this capability if you want to hide things from the owner of the computer.

It's not like they are hiding anything - "trusted applications" has been a dog whistle for DRM at least since "Palladium" (later called "Next-Generation Secure Computing Base"), part of Microsoft's "Trustworthy Computing" plans ~15 years ago.

[1] https://software.intel.com/en-us/blogs/2013/09/26/protecting...

Aren't there also legitimate use cases for these things? E.g. a password manager loading itself into a confidential memory region to prevent infiltration?

That may be, bit I'm absolutely confident it is perfect for malware and spyware too.