When should I use Avacyn's Glory's Restoration Angel?
Many cards have an ability that triggers when coming into play.  With Restoration Angel, you could re-trigger one such ability.

For the deck you mentioned, Goldnight Commander has the following ability:


  Whenever another creature enters the battlefield under your control, creatures you control get +1/+1 until end of turn


So, when you play Restoration Angel and remove any other creature, your creatures get +2/+2 instead of just +1/+1.  If that other creature is Geist-Honored Monk then the boost to all creatures becomes +4/+4.

This is just one example.  Fiend Hunter, Champion of the Parish, Mentor of the Meek, and Soul Warden could also be used with this strategy.

Alternatively, you can dodge spells and abilities this way.  If a non-angel you have is targeted by a spell or ability (say Shock) then removing the target from play temporarily will cause that spell/ability to miss and do nothing instead.  As @skovacs mentioned in the comments, this can also apply to unwanted enchantments on your non-angel creatures.