Flats are more comfy but i dont really feel that dressed up unless i have heels on. Maybe comfortable to wear, but heels & boots are sexier to look at 
Since high heels is not that healthy for your feet and body, so there must be time when you change if for flat shoes, so when ? For me, those are the times that I wore flat shoes.
In my casual time when there is not any important meet up with somebody.
Go shopping with my friends and should be a lot walking during the day.
Outdoor activities like mountain climbing, cycling events or just trekking.
Flat shoes can sometimes be sexy too based on some clever designs.
At home definitely.
