To quote and this is on the home page from the video.
alsoThree difficulties plus Raid Finder means that there is a raid difficulty for all of the different groups. Normal for friends, family, and PuGs. Heroic for guilds that are formed for the purpose of raiding, Mythic for the extremely hardcore players.
Incoming anger in 3....2....1The team was worried that LFR was growing too much in prominence in some ways, so they wanted to make sure that players were doing regular group raiding didn't also feel obligated to do lots of Raid Finder to complement that. The team probably overdid this in Warlords by removing Tier sets from Raid Finder, so they are coming back to Raid Finder in Legion.