Just yesterday when leaving Thailand from Don Muang airport I was asked at iimmigration to speak to the supervisor, who asked me to fill out a form with details of my stay...  like what the purpose of my trip was, where I was staying, phone number, etc

I was on a 30 day extension of a tourist visa, leaving 3 days before it expired.

Has anyone had to do that before? When leaving the country? Why do they take this info from you?

Thanks... I'm just worried about my reentry within two weeks

Sounds to me like you were asked to complete the dreaded Foreign National Information form which has been mentioned in several recent threads on here. If this was, indeed, the case, this would by itself be unlikely to affect your re-entry in a fortnight's time, I would have thought.