Buying things get you stuff to sell later down the line for profit, routes are shown from maps and clothes help keep your "master" feeling well.

Also, in addition to what DocRask mentioned, sometimes clothes can be used to negotiate an earlier departure date without paying exorbitant fees - especially if you have a complete set of the correct attire.

Buying timetables automatically opens up new routes (unless you've already discovered them). Note that unless you discover those routes via some other means, selling the timetable removes the routes from the map!

If you hover over objects you'll get a description of what they're for. More importantly, you should be notified of where the specific object can be sold for a profit, and how much the object is worth in the current location. In the case of the really valuable items, you should also be notified of how much you can sell it for at the correct destination - which for those objects can be thousands of pounds profit.