I have a couple of Shimano wheels with 11 speed cassettes.
If I buy a bike with 9200 I want to be able to use them. I am not going to buy 12 speed cassettes when I have half a dozen 11 speed cassettes in the basement.
If I break a 9200 rear derailleur I want to be able to put any di2 derailleur on as replacement. The exact replacement is not always available.
I have a couple of Shimano wheels with 11 speed cassettes.
If I buy a bike with 9200 I want to be able to use them. I am not going to buy 12 speed cassettes when I have half a dozen 11 speed cassettes in the basement.
If I break a 9200 rear derailleur I want to be able to put any di2 derailleur on as replacement. The exact replacement is not always available.