You switch between the two options here:
Enter Map Options to get a variety of choices that will blow your mind (hmmm not so many but still useful and worth checking).
Tick the Streets and Highways box.
Compare the maps of London.
(Click to enlarge)
No idea if anybody needs such detailed maps but just in case, if you're wondering how the maps look when they are enlarged:
For Tableau Classic maps you can switch on Place Names, then the names of churches, universities, museums, etc. appear on your map.
It can be useful or annoying, everything depends on the situation, as additional text on the map can be distraction from the data.
Below map of Moscow with the Place Names.
If you aren't happy with built-in maps you can check Allan's tutorials for applying other maps (It's for Tableau Desktop).