Note: This is just AFAIK.
Since you don’t care if a value type is handled by the garbage collection, the only difference between the two is that the non-uproperty version is no longer available to blueprints of that class.
If you mark it as UPROPERTY, blueprint can see it and interact with it depending on what specifiers you give it.
If you don’t mark it as UPROPERTY, you’d have to create UFUNCTION() getters and setters to interact with it.