can someone explain, why we cant use one class to do finish this challenge? Why we should have 2 different class?

Good question. It is entirely possible to solve this in a single class. In this case, I would name the Hand class specific to the type of Game Hand.

It is good practice to divide up object structure at hierarchical boundaries where the attributes and methods at each level would operate differently. For the card, it could be a methods to select values and compare itself to other cards. For the hand, the method to compare to other hands would be different that the card to card comparisons. The hands would also have methods to generate new hands, select cards, and so on.

As an extension, the card and the hand class could be more generic. Then you could create a derivative hand class that would inherit from the base Hand class, such as, PokerHand, Crazy8sHand, Pinochle, etc, Also the Cards could take a Deck class argument that would specify the range of possible cards. This deck class could have a shuffle method and a __next__ method offer up the next card in the deck.