Bootstrap grid

Question

The bootstrap is responsive css library. right?
Then why do we need add xs,sm,md,lg to the end of the col class.As they automatically get their size resized for different screens.

Answer 1

You define how your website looks on various sizes of viewports with or without Bootstrap's help. Bootstrap doesn't make all the layout decisions for you.

When viewports get narrow enough to be shown on a phone, most websites wrap content but unless you manually test and review it, what content wraps, where, when, how... can often look pretty bad. Indicating how you want the layout changed for extremely small, small, medium, and large viewports is what xs, sm, md, lg are for.