The suffix -y is used for the Dative case, which corresponds to English "to" in most cases. I gave the book TO MY BROTHER = я дал книгу моему брату. Some verbs require the dative, where English doesn't. For example, it is used with помочь 'to help' : I help MY BROTHER = я помогаю моему брату.

Strictly speaking this is not a 'suffix', this is an ending. Word endings are more volatile, unlike suffixes which do not change by case.

В примере это Дательный падеж, кому? чему?