To allow somebody to change how the encryption between a server and client is handled so the communication can be intercepted. Either by putting a thumb on the scale for the cipher used or outright using a particular key that is known by an attacker.

E.g. to make a man in the middle or even passive traffic capture attack possible so they can choose to intercept supposedly secure traffic when they want to

I don’t know if the full extent of what it can be used for it known yet. Just that how they hook the calls for the traffic allows for some pretty nasty scenarios.

I literally just gave you a list of what's vulnerable: Debian / RPM -based distros on x86_64 Linux that installed new versions of liblzma5 (which are so new that likely only rolling release distros had them), running OpenSSH sshd via systemd.

As to what it could be used to do and what its purpose is, well, the backdoor's still being analyzed. Seems to be for remote code execution, ie. the attacker could theoretically execute code on a backdoored machine.