The flair fitting holds the toilet to the floor. The holes provided for the nuts and bolts help in exactly fixing the toilet so there is no leakage of drains. Some amount of space is left between the wall and the flange, so the toilet is fixed exactly. In case you are using a floor joist then the toilet size needs to be either 10 inches or 14 inches. If you are highly economic, you must prefer what is called a 12 inches offset flange.

A Dry Run
Toilet bowls should be installed so that they are level side to side and front to back. The trouble is most bathroom floors are not level. Once your flange extender is in place I want you to set the toilet on top of the flange without the wax ring in place. Use wood shims and a two foot long level to get the toilet so that it is indeed level.