[rossvor]

I personally never experienced it myself. But I can imagine it being useful in a relativistic sense. For example if the candidate puts 9 for PHP and 5 for Haskell, you can get the rough idea what language they prefer or are more comfortable in using. You are not comparing these ratings against other candidate's ratings.

[cutemonster]

Precisely -- one's estimates about a skill, are accurate (only) when compared with one's own other estimates about oneself.

And can be helpful when preparing for an interview (for the interviewer).

I find it surprising this didn't occur to the other commenters