People mostly travel to know more about the culture of some other place. The culture includes everything that is depicting a particular aspect of an area. A museum itself depicts the different eras through which an area passes. If there has not been a museum to showcase the different aspects of a place, how a tourist come to know about the place. Unless he learns something about the place the whole purpose of the travel and tourism will be lost.
Even if we take the leisure tourism or adventure tourism we will find that an area is promoting itself showcasing its culture. And the motive of a museum is also to promote the culture. It is for this reason a museum is considered to be an indispensable aspect of travel and tourism.