You really want to be able to cast your cards, so if your deck is full of high-CMC cards, you must build the rest of the deck to help do that
Giving your creature indestructible can be a lifesaver if you need other stax pieces (I am assuming if you play Boromir, Warden of the Tower , you are playing a stax deck) to stay alive, or if you just want to beat down someone and survive a board wipe
A 3/4 angel with flying that costs one red and two white, her entire gimmick is that any spell you cast that targets a creature you control gets exiled once it resolves
Its still a work in progress discovering which mix is best
No, some are just pointy-eared mana finders like Greenseeker, or blunt-eared mana reanimators like Groundskeeper or even Snake-eared mana boosters like Sakura-Tribe Scout
Hexproof protects against an opponent targeting my permanent with their spells or abilities