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.

With the Premium plan, you get 30 credits a month
With the Business plan, you get 300 credits a month

Find out what credits cost here.