It's legal, at least in the US, if there is a dotted or broken yellow line between the two lanes, the opposite lane is clear, and you have enough time/space to complete the overtaking before it swaps back to solid yellow. For obvious reasons, you generally want to spend as little time as possible in a lane that has traffic coming at you head-on.

>Also why is it safer?

Because you spend less time and less distance in the "wrong" lane.

Which you should not have entered in the first place.

Your opinion is asinine. If using the lane at that time/place to pass was not suitable it wouldn't be signed/marked for such. Increasing the time it takes for people to execute a pass is a regression in safety from the status quo.