Specifically secure boot is what makes it so that "your" computer is unwilling to run software that has not been approved by the company that made it. This has existed for quite some time, and is responsible for the locked down mobile ecosystem as well as the inability to remove the Intel ME and AMD PSP embedded malware from recent PCs.
Remote attestation has not been widely implemented yet, but will make it so that remote services refuse to work unless you are running only software that the service approves of. I'm not sure how much Pluton moves the needle forward, but any amount is not good. If remote attestation comes into full effect, many websites will only be usable on newer computers and websites will be able to forcibly disable software the website finds objectionable, like say Adblock.
A lot. They only need to wait for Pluton enabled PCs to reach critical mass. Compared to TPM's, Pluton is inside the chip thus not vulnerable to bus tampering and is not a standard but a "product", meaning Microsoft will have the ability to make changes without intervention from other companies.