The spirit of when you may take actions

[h0f1]

It makes sense why items can not be switched out during combat but what's the idea behind playing race, class, items, or selling items only on your turn?

Are there cards that make use of this rule? Are there cases where this rule has any effect on the game? Are there any cons to playing those cards at any time? I find it much easier to play those cards whenever I want.

Also when the game starts the rules states I can play any race or class from my initial 8 cards. The next sentence says I can play any useable items. The next section of the rules says to roll a die to determine who goes first. Do players get to play their cards before the first turn or do they have to wait their turn to play any items?