You wanted a source OP, here it is directly from WoW lead designer :
" LFR justifies the creation of more raid content when millions of players are able to see content. Only a few thousand people actually saw Kel’thuzad, but millions saw Deathwing. The reason Mists of Pandaria is starting with 18 bosses and adding larger raid tiers than we have had previously is because many players are going to see the raids through LFR. "
And I have to say that as someone who is a veteran of the game (which is easily noticeable if you look my achievements how that’s true), and as someone who raided back in vanilla and TBC and experienced Sunwell back than when it was the current content, the raid which only small percentage of the playerbase experienced back than, and as someone to who the LFR was never the main mode of the raiding and who wouldn’t be affected if it would be removed how I’m glad that it exist for all the people which wouldn’t otherwise without it be able to see the raiding content for whatever reason.
Also for all the naysayers which say how the LFR is bad for the game and how the players can’t get improved in it there are threads like this :
https://us.forums.blizzard.com/en/wow/t/how-i-went-from-lfr-hero-to-mythic-raiding/38715