There are plenty of reasons why MPP databases allow the definition of constraints but don't enforce them. I'll list two: 1) BI tools can use them to optimize joins 2) Data modeling tools can use them to reverse engineers models without having to pattern match the keys.

That said, Snowflake does support constraints if you use hybrid tables (a preview feature announced at their last conference).

Metadata

Tools and scripts can work off of it, design decisions are documented, suggestions can be made, inferences can be made (some dangerous, some not).

Why tag S3 objects if it doesn't enforce a schema? Maybe a bad analogue but I'm going quick right now :).