It's used in Rails to reduce the likelihood of un-sanitized user input in SQL fragments [1]. I think it would see a lot more use if additional input sources were marked as tainted [2].

[1] https://api.rubyonrails.org/classes/ActiveRecord/Base.html#c...

[2] http://www.jkfill.com/2012/03/10/preventing-mass-assignment-...