Making the distinction serves no purpose other than the expression of ideology. Transgender identity is already on the list of protected characteristics, so defining sex as biological has no effect on who is protected. Both cis women and trans are protected whether sex is "biological" or not.
Also, this cements "biological sex" in Scottish law so when the Supreme Court ruling is overturned, there will be this biological sex in another act. Presumably the religions zealots will be attempting to do this with other legislation.
"Biological sex" is theological sex by the way.
I wasn’t aware I could know anyone else’s biological sex without a lot of chemistry and nudity.