In Search Match Ad Groups default CPT bid is very important, as it is the actual CPT bid that will be used to determine wether you will win an auction with a certain search term.
So in your discovery campaigns in ad groups with search match on, Apple will use your default CPT bid set for this Ad Group.
Note that in all other Ad Groups that contain keywords (Broad or Exact Match), the bids of individual keywords will override the Ad Group default CPT bid.
However, when you add new keywords to an Ad Group, they will be added with the default CPT bid for the Ad Group unless you tick the box to change the CPT bids manually.
Moreover, in ad groups where you have many keywords with different bids, default CPT bid can serve as a quick reminder for you of what bids you have on average for the keywords in that ad group.