User Profiles store site specific information. This is information is then stored in the FITs header of your saved images. In general, this information is useful for astrometry measurement (or even for personal organization).
Selecting a profile as "default" means that all new sequences will automatically use this profile.
Auto Populate: If you have an Internet connection, you can attempt to auto locate your position. Please be careful with this... sometime internet geoloaction is not so great.
Additionally, if you have a telescope (mount) connected, you can either: