I see a lot of people using this card, can someone tell me what, or why are they using this?

I would imagine that you're seeing Phoenix Blade most commonly in Diamond Dude Turbo lists.
- In this deck, Phoenix Blade provides some minimal pump, which is nice at times, but it's playable because of it's RFP clause, and the fact that it returns to your hand.
- This allows DDT to fuel their mass Return effects, to win the game, if they need to use that route, and it allows Phoenix Blade to become a solid pitch-fodder card, for things like Magical Stone Excavation, or any other multitude of discard-cost effects that would be run, either in DDT, or another archetype.