A barometer is a sensor for air pressure/weight/density. It measures that one thing, and tells you two different things (sea-level air pressure, and elevation above MSL) from that. One dial, two sets of numbers. For that to work properly, it needs to know what the current conditions are. Telling it either the current altitude or the sea-level pressure gives it the information it needs to work properly. This is something that has to be done from time to time (after conditions have changed) for an altimeter to work the way it's supposed to.

Any altimeter can have issues in certain conditions. Especially if the air pressure changes while you're moving, that will get you wrong relative numbers, not just wrong absolute ones. That's why Suunto and Garmin developed systems to continuously re-calibrate a barometer from GPS data during an activity. Calibrating when necessary reduces or eliminates those issues.