Our user Nicolai Wolpert contributed particularly impressive pictures of the 1900 World's Fair in Paris [3]. See the comparison of the Quai des Nations in 1900 and 2017 [4] if you are in a hurry.
We feel that rephotography is a great tool to make history more tangible to a greater audience. Sometimes it's difficult to visualize that and how people lived in the past; seeing those two pictures taken in different times makes changes (and consistency) in peoples' lives much more accessible. When taking after pictures this effect is even more pronounced. It's fascinating to know that some photographer stood in the same place decades or a century ago. ;-)
This is mostly a passion-driven project at the moment, so it might be rough around the edges in some places. In particular there isn't a mobile view yet for most functionality, so I'd recommend using a larger screen.
[2] https://en.wikipedia.org/wiki/Rephotography