Dashes in locations and dates in old novels (e.g. Jane Eyre)?

It's partly what Marli says, so that the reader can't prove the writer wrong. (Research was much more difficult and expensive in those days than it is now.)

It's partly to allow for the pretense that the story is actually true, and that the names have been omitted to protect the innocent, or to prevent the guilty from suing. Back then, many people thought that reading fiction was a waste of time, or even harmful. (Rather like the moral panics over certain types of films, or certain types of music, or certain types of computer games. Nothing new under the sun...)

Those dashes irk me when I see them. 

I think either the author or publisher back then did not want complains from readers that "Mr Anthony Trollope is mistaken in claiming that his character Mr. Phineas Finn was Member of Parliament for our borough in 1853." or that "There was never a scoundrel such as Mr. Wickham in our regiment and we will sue you if you do not withdraw 'Pride and Prejudice' from publication at once!" Military blokes were touchy about protecting "the honour of the regiment". 

The names and dates are left blank because the specific name and date isn't integral to the story. The dashes serve to make things ambiguous, which in some instances is a much better option than going with an actual place name or an actual year.