I know the theoretical range for each voice type, but I would like to understand know the real ranges of singers in practice.
Those are the estimated ranges for each of the voice ranges. From there on, each individual singer will have its own range.
If you're a singer, do you have the same range as other singers with the same voice type?
You'll have similar ranges. Some singers of a voice type will get a little higher, some a little lower; the reference is some kind of 'mean' of the ranges. Each individual will have its own range, which will be unique to the specific singer.
does it even make sense to know you exact range?? How flexible can it be?
As of flexibility, given a range from a singer, it can be expanded with correct training. After years of professional training, the singer gets to know the range where he/she can sing most comfortable and will, therefore, dedicate to the repertoire in that range. In that sense, it not only makes sense to know your exact range but it's what a singer naturally needs to know so he/she can keep improving and know which repertoire to perform.
Hope this helps!