In my application, users are entering a coordinate in a fancy "corrected for all sorts of stuff format". Any map people, you know what I mean!
The entries are not required for the form.
The entries must be of certain character length (seven digits in my case) and be between an upper and lower bound. I would like to use validation, because once entered, the value must be correct. It's too easy for people to enter the wrong number of digits or enter a number so clearly wrong.
When using validation rules to achieve this, the value effectively becomes required because it won't submit the form with an empty field. The numerical field with no entry is treated as a zero, which falls outside the allowed limits.
It would be much cleaner if an empty field was treated as NULL.