Using credits is only applicable to iOS users.
Credits aren’t a necessity, you only need them if you’re using the Auto Send feature. That's because when you send a message through Auto Send, it is sent through a SMS gateway, which requires a small fee. You buy credits so we can pay that fee. Without it, we couldn’t offer you the Auto Send feature. We wish we could change this, but sadly we are not wizards.
Find out what credits cost here.