Actually, the point is to manage expectations. Instead of saying I'll be there in 5 minutes, it would be better to say I am going to be X minutes late. There's lying to hide your business, but there's also lying and being inconsiderate of other people's time.

Or, it could be your best estimate, based on current traffic conditions or walking speed.

I always underestimate how long it will take,this does not mean I am lying, it means I am bad at guessing how long it takes me to walk/drive/sit in a train somewhere

In which case surely your friend getting an automatic, accurate version of your position from facebook would be better for both of you?