What's the point of Divine Smite as a reaction?

Hey, new to playing Paladin.

What's the point of using a reaction to cast Divine Smite, when you can cast it normally like other smites? Doesn't it use a reaction for basically nothing as you could have saved it if you just cast the smite normally? Or doesn't it in fact use a reaction point? But then what's the difference between using it as a reaction vs a normal action?

I'm confused