https://en.wikipedia.org/wiki/Downforce

Yes, and at high speed, it helps make sure you don't take flight accidentally.

When you don't have enough:

https://www.youtube.com/watch?v=e21ZjwZGjiQ

It helps everywhere... turns and straights - it provides downforce without wings. Wings cause drag, that push the car down but also slow it down. So the car can be going slow and still get incredible grip. On the corners the car won't slide, so it can corner at much higher speeds too. For both these reasons, this suction system was banned by most racing leagues in the 70's for being too powerful.

It's for more traction. Not a road car, but this one does 0-100kph in just under 1s. https://www.youtube.com/watch?v=XQIu5tZ0vbQ