Benefit from Mobile Pos terminals?

Question

Payment terminals

Hello, now I am considering a payment system and came across mobile payment terminals, such as _2can
So I thought about it, if I have an online store and I come to a client, he said that he would pay with a bank card, and when I arrived, he paid in cash.
And according to the law of the Russian Federation, I have to give him a check, which will indicate the time of receipt of the goods. If the check is issued earlier or later, then there is a fine from the online store.
So why then do these mobile terminals exist, if you can buy a mobile card that can issue a check - in cash, and swipe the card?
Or do I not think so?