They serve to give your side of the board tempo by allowing your creatures to attack (removing defenders), giving you more time to set up the board (removing attackers/expensive creatures), and allow for combat tricks (saving your creatures after damage is on the stack
On the opposite side of the spectrum, Orphan, Cocoon falCie is the beautiful and sinister Muldrotha the Gravetide reprint I never knew I wanted
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of The Wind Crystal's first ability)
This is especially true in cEDH, where this card looks like an absolute slam-dunk
All you need is a few more red permanents and suddenly youre drawing a whole new hand