1. No difference. The module slot on motherboards is there only because older CPUs didn't have such capability so anyone who needed TPM could easily add it in as a separate hardware gadget. Also, it's cheaper to add the module instead of replacing CPU if the CPU works like a charm except it's TPM part that farted.
2. No unless you enable Bitlocker on the system drive, set PIN for it and then forget PIN in which case the data will be inaccessible and you will need to format the drive (reinstall OS).
In other words, don't use Bitlocker on the system drive unless for some reasons of your own, dunno what could that be, you have to. You can use it on USB drives though so in case those get stolen, a thief can't read anything because they don't have PIN.
3. Noone knows why exactly Microsoft decided that TPM is required in the new OS for everyone. I can understand a security benefit at workplaces, but for a home user it should IMO be a choice, not a requirement.
On the other hand, it might be a step into virtual smart card tech where certificates usually kept on plastic cards can be stored on TPM chip instead.
What does this mean? You buy some software and install it, but the license goes as a certificate on TPM, it is not written on your disk. In case you have to reinstall everything, no need to search where the hell was that license as TPM is not erased by reinstalling.
Note that TPM storage is limited so you can't have TPM certificates for all of the steam catalogue.
In short?
Do not buy the module.
Enable it in BIOS (UEFI actually, but everyone still says BIOS) if there is such option and you want to switch to win11.
If no option, keep using win10.
