In chapters 21 and 22 of Frankenstein, by Mary Shelley, we are placed at the time of Victor's accusation and trial after the death of Henry Clerval, his best friend, who was killed and dumped at the beach by the monster.
Victor was the first suspect in the death and was accused almost immediately until the local magistrate was able to prove that Victor was nowhere near Clerval when the murder occurred. However, this still left a lethal mark in Victor, who spent two months with a comatose nervous breakdown as a result of the ordeal.
Because of that, Alphonse Frankenstein, Victor's father, decides to take Victor with him to take care of some personal businesses he had going on in Paris. However, this was also an opportunity for Victor to confess to his father what he had done. He took sole responsibility of what the monster did. However, Alphonse wouldn't hear of it and assumed that his son was just overly exhausted due to the situation in Ireland.
Concisely, Alphonse took his son with him to Paris as a way to alleviate his son's delicate state of mind, and to be next to him while he went to conduct businesses in Paris. He also wanted to take care of his son, for whom he felt very sad and sorry.