Each list can have its own unique branding, like a different events or products. Therefore, we suggest attaching a logo to each list you set up in order to reinforce the branding. It is not, however, required for you to have a list with a logo.
Your logo will be placed on any auto responder you create for that list, any subscriber form you create and even the Unsubscribe form that subscribers see when they choose to unsubscribe.