Why do scientists classif different species into th same genus?

A genus is a part of a family and each genus contains animals which are very closely related.

genus or genes as we can also say contains animals which look the same and have the same charectaristics , but can be told apart because of their colours , and are therefore sub divided into species.

For example :

a red eyed tree frog and an eurasian green tree frog belong to the same genus - frogs but are of different species.