I have researched this for the answer and have yet to come across the "Why".
You can clone from any plant so why keep a mother plant instead of just cloning off your current vegging plants? I have read research where recloning does NOT lower yields nor THC. So why keep a mother plant?
A clone is a clone...so it's always an exact replica
Like somebody posted before,it depends on the situation.
I started my grow with one sweet tooth seed,so obviously I had to grow it out to be a mother plant.
Then after I took cuttings and it was time to flower I saved one of the cuttings to become the new mother and flowered the old mother with the rest.That saved a little bit of space in the veg room.
I like to veg my plants for a rather short amount of time and by the time I put them into flower they may not have enough branches for me to take cuttings from them.
On top of that,if you do take cuttings from some plants rights before flowering them.It forces you to veg these cuttings for however long it takes the others to flower(at least 2 months).And thats way longer than I ever veg,unless growing out a mother plant.
Also,over time plants can mutate.So,yes a clone is genetically identical to it's mother,but that doesnt mean that it's not different than the mother was 20 years ago.