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Everything in Supernatural has a consequence. At the end of "Death's Door," we were left to wonder Bobby's choice. At the end of "Party on Garth," we saw that he had chosen to stay. In "Of Grave Importance, we---and Bobby---learn what those consequences might be.
The boys receive a call from a fellow hunter named Annie. She expresses her condolences, but also mentions that she has a few of Bobby's books. She sets up an exchange, and they decide to meet. Unfortunately, she doesn't make it to that rendezvous. Annie is working a job, and in the process, ends up getting herself killed. Sam and Dean are left in the lurch, worrying that something might have happened to her.
They head to the site they know she had been working her job at, and set out to investigate. Unknown to them---as he has been for some time---Bobby is also tagging along. He runs into Annie's spirit, confirming the fears that the Winchesters already have. Upon return to the hotel to start piecing the case together, Bobby desperately tries to communicate. He writes in a mirror that Annie is trapped in the house---and Dean is stunned. Once Sam enters the room, Bobby writes his name to confirm to them that it is indeed him.
Once they return to the house, Bobby in tow connected to the flask, he and Annie try to work the case from their side. They also desire a way to reach out to Sam and Dean, and so they watch another spirit---Haskel Crane---easily manage to move a chair and sit down at the bar. He scoffs at them for being novices---and forewarns both of their future as spirits.
Crane tells them there are two ways to move objects---through calm or through extreme anger. Most spirits on Supernatural favor the latter---bringing hunters to vanquish them often. As a spirit gives into that anger to move objects, it changes them, and as they stand talking to him, another spirit flies at them in a powerful rage. She screams and speeds at them. Her expression is marked by vast fury. Whatever semblance of sanity she may have had has long gone. The spirit explains that this house has numerous spirits---and that spirits decay at different rates. Some become vengeful quicker. Others turn more zombie like---as the other spirit they see stands still and vacantly staring, the flesh on its spectral face rotting away.
No matter what, much like a star burning through its fuel, this is the fate of all trapped spirits on the Earthly plane.
Someday Bobby, too, will have to face these consequences. His choice is rife with good intentions---but as good intentions often go on Supernatural, they always seem to bite one in the behind. Sam learned the hard way in his pursuit of Lilith. Castiel faced the same fate upon releasing the Leviathan from Purgatory. Now Bobby must deal with the consequences of his good intentions.
Yet, despite this dire future prediction, there is a layer of hope. Dean expresses his fears to Sam, stating, "It's not the natural order," and while it isn't, Bobby has so far beaten the odds against him. He doesn't seem trapped in any death echoes. He's not fixated on someone who had done him wrong---only to turn that outward on all that resemble in any way his killer. Certainly, Bobby wants to destroy Dick Roman, but so far it seems that his goal is more to stop a monster instead of avenging his own death. Most importantly, Bobby's fixation is that on helping his boys---something he did in life.
His boys. That's the only reason Bobby chose to stay at all. Annie even reminds him of it when she states, "You have the boys," after she expresses her desire to be put to rest. They were the last thing he saw in "Death's Door," and they are the reason he still remains with them on earth. His reaper told him that he had done enough---but Bobby feels that there is too much unfinished business for him to simply move on. He doesn't want to leave his boys behind to deal with this alone---and it is that love that shines through this episode beautifully.
Even while spectral, Bobby does all he can to guide and support Sam and Dean. He wants them to have all the clues they need---but he needs a way to give them to them. He wants to be a part of this fight---as much as he's always wanted to be a part of this fight. Being left behind by his boys is not something he desires. We've seen it time and time again throughout the series. When Dean tries to leave Bobby behind to go after Lilith before his deal is due, Bobby quips, "Do I look like a ditchable prom date to you?"
Bobby wants, more than anything, for Sam and Dean to know that they are not alone in this fight. Be it Azazel, Lilith, Lucifer---or now Dick Roman, Bobby won't abandon them. This could be in part due to his witnessing John doing much the same to the boys---or it could be connected to his guilt for not giving Karen children. He is determined to aid them, to guide them, and to support them in any capacity that he can provide. As a spirit, that directive has not changed---and that is a good thing.
Certainly, Bobby will have to let go---if he's not returned to the living of course---and move on. Sam and Dean will not live forever, and when they die for good, Bobby could potentially be left behind to morph into any of the vengeful spirits he encountered in the house.