I am kinda curious, about this.
Is it really necessary to make a subsidary, if I am based in London, and I also want to operate flights out of lets say Amsterdam? I do know, that my Amsterdam flights only can operate within EU - but do I have to operate a subsidary?
Thanks in advance 
(And yes, I did read the wiki, but didn’t find an answer to my question 
)
No, you don’t have to have any subsidiaries. It is simply easier sometimes to separate different kinds of operations, especially if they use different plane types - you save on maintenance that way and can have different maintenance providers.
As Elladan has already said, a separate subsidary can be useful if you want to start a cargo carrier. You can put your subsidary up to stock market (you can’t with holdings) and gain extra capital (giving 20% of your shares to other investors in exchange). Also, as explained in AS wiki, you will have unlimited rights/assess (same to those granted to domestic holdings) where you start a new company in a investment-open country.