This was introduced on September 5th, 2014.
This is live across the entire Stack Exchange network.
I've used it once. The motivation for providing this options stems from the ease with which certain insignificant edits are considered helpful when the moderation requires additional edits. That is, when someone makes an edit that the moderator (anyone with ...
You're right, line spacing has been increased as mentioned here on Meta Stack Exchange: New post formatting
Many people don't like the change, and posts like Please revert the line-height change! and this answer are very popular right now.
Previously, different sites within the network may have had their own code base, even if it were subtle. Changes like this seem to bring sites like ours in-line with the rest of the network. This promotes consistency across sites and is far better for site improvements.
Most changes to the network are posted here: Recent feature changes to Stack Exchange