If you update a row without actually changing its contents, does it still hurt?
Paul White wrote in detail about the impact of non-updating updates, proving that SQL Server works hard to avoid doing extra work where it can. That’s a great post, and you should read it.
But foreign keys add another level of complexity. If you have foreign keys, and you tell SQL Server that you’re updating the contents of those related columns, SQL Server will check ’em even when the data isn’t changing.
To illustrate it, I’ll take the Stack Overflow database and add a foreign key