Because Criminal Minds is one of the best crime shows and Emily Prentiss is an inexpendable member that both the BAU team, the cast and us fans lost.
They say that they feel as if they have lost a limb, that the team will also be in grieving, but I believe us fans (especially those who havebeen with Emily since her first appearance in Season 2) are also in mourning too. It feels weird to imagine the show without her.
The episode had its pros and cons; I'm still going to say that I mostly liked it despite its weaknesses because the acting was amazing, and it still managed to make me feel something and go teary (especially the scene where Emily hears Garcia's voice message).
Tribute to Paget Brewster/Emily Prentiss. The past that haunts her and the present that will soon become her past.
The quote is by Kurt Vonnegut; 'You are what you pretend to be, so be careful what you pretend to be'.