Lukáš Svoboda wrote another excellent post on his blog that explains the true motive for numbering houses in 1770 . It is in Czech, but you can easily open it in the chrome browser, right click, and select "Translate to English." Or copy and paste the text and dump it into google translate.
In short: the legend that Maria Theresa did it out of frustration after wandering in Vienna one night, looking for a house sign is not true - or at least the whole truth. It was done in part to aid in enumerating the citizens of each village and thereby establish a more equitable military draft.
Why then? What was the impetus for Maria Theresa to go to all that trouble funding it then? It was to prevent the Seven Year's War from happening again.
Another interesting thing that I learned from this blog post was that the house numbering itself was a big event, with lots of planning done by the parish priest, military officials, soldiers, and citizens. I would love to understand that exact process more, so I plan on asking Lukáš if he might write a guest post about it soon.