How were travel and travellers important in Shelley's time?
In the days before radio and television and the internet (and before most people were able to afford much in the way of books), travel and travelers would have been an important source of knowledge about other places and other people.
You can see this, for example, in the way that people who travelled to some foreign place would often publish books about their experiences. A famous example (from a bit after Shelley's time) was Alexis de Tocqueville, but he was by no means the first or only person to write books based on what he observed about a foreign country.
So travellers spread knowledge of their own culture and ways and they collected knowledge of other countries' cultures and ways.
Shelly wrote Frankenstein in 1818. This point is important to keep in mind, because this is was when England was growing greatly in overseas colonization and exploration. There was a buzz in the air for exotic voyages and travel. So, the general population absolutely loved to read about these things in books or hear about these things from explores. Shortly after this time we would see the rise of missionary work as well as cultural anthropology. So, to answer your question, travel was very important, not only for trade and commerce, but to explore the world and experiences other cultures.