The Dexcom gets an electrical reading from the wire and needs to show us a result in units we can understand. If the ratio between the electrical reading and actually blood sugar is changing (as it may as sensors get older) than a calibration is helpful. I've never found these changes to be fast, so I generally don't care to change this ratio until I need to.
Some feel that frequent calibration will mean more accurate results, but in reality I think a lot of frustrated Dexcom users that post here are calibrating when they aren't level, and get flustered when the results diverge from their meter later in the day.
For me, calibrations are something that I do very infrequently. I usually go days between calibrations. I'm still checking my blood sugar with a meter, but unless the results aren't close, I've found that entering more data than is required only leads to variable accuracy.
Also, if the Dexcom reads 100, and you enter a calibration of 100, it will likely tell you that you are now 90. Something in their software seems to think this is best, but I see no need to add extra jumps that aren't necessary.