Casual PVP, unlike Ranked PVP, is, well, casual. It is best used as a testing ground for untried teams, before taking them to the Ranked battlefield.
You don't win or lose PVP Points for playing it (or being invaded by others playing it) and you gain less rewards for winning (less gold, limit of 1 trophy per match). Casual PVP matches do not contribute to Ranked PVP stats in any way.
An important note is that, unlike in Ranked, the difficulty levels Normal, Hard and Warlord I-IV are applied in Casual PVP, meaning it is possible to increase the default rewards by using a higher difficulty than Ranked allows.