3) Why do rangers cast spells?
Because in core D&D, the rules for the above don't exist or don't scale past level 5. So designers just copy and reuse spells.
I disagree with this. I think spells are a core part of the Ranger design concept and not a result of the features they are trying to implement. I think spells on their own are at least as necessary to the class identity as some of the things you mention.
Also the 14 things you list are covered by spell casting, but many of them are not covered well by the Ranger spell list and you need to go to other lists to get some of them. Which brings me to my next point - Ranger spells used to include spells from the Magic user (i.e. Wizard) list and included a spell book in the same fashion as a Magic-User. Wideing the current Ranger list to include Wizard spells would improve the class identity and in fact adequately achieve more of the 14 things you mention.