Lara can turn around quickly using the roll button. While standing or running, Lara will make a roll and end up facing the other direction, almost one block-length away from her starting position. She will need almost one square to do so. For example if you pull up onto a 1-square block and make a roll there is just enough space for Lara not to fall off.
This move is also useful when you have a timed switch. After flipping the switch on a wall immediately roll to turn around and face away from the wall.
It is also useful when fighting baddies. Run past them and roll to shoot them in the back.
Since Tomb Raider II you could also press the Roll Button while under water, to turn Lara.
In some cases you will have to use the roll button for other actions. For example you can use it along with left/right to get off a vehicle (like the Quad Bike). Or it can be used in The Angel of Darkness to change the target.
In the later games the roll has been modified and combined with the Dive Roll.