פירוש על שבת 42:9

Another light: To make the thing noticeable.

But if there is a bonfire: A fire. [Then] there is no need for another light, since one will use the light of the bonfire, so it will be noticeable that the [Channukah} light is for the commandment.

But if he is an important person who is not accustomed to using the light of a bonfire...

He must have another light: For if not, it would not be noticeable.