Session Reports can't be linked directly to Plots. They can only be linked to Sessions, which require an active campaign managed by World Anvil.
I have tried several times to use the campaign system, but it's just too much for my needs, considering the small scale of campaign and player group. The ability to take this lighter-weight approach to session tracking would be greatly appreciated.
By allowing Session Reports and Plots to be directly used with each other, the World Anvil campaign system becomes optional: A power tool for users who need that functionality, but not mandatory for more casual applications.
None that I can think of.