Ok, so I have a theory about the way spirits and death work.
People who led a good life and were decent enough to their fellow men, like the old man, who have no regrets/unfinished business are granted some free time to do whatever. Like deliver a message in the case of the old man. And then they move on to the afterlife.
Spirits with unfinished business get more corrupted the more time they remain, but are generally not malicious, like that businessman with the safe code from 20 or so chapters back. After their business is concluded, they can finally move on, but are unable without an assistance due to sustained corruption, hence why their corrupted forms have to be eaten to be cleansed. After which they move on to the afterlife. Maybe. Probably. Hopefully.
Then theres the spirits who have unfinished business and said business can no longer be fulfilled or the spirit has lingered on for far too long - those are forcibly removed by the entities like the cafe spirit.
Then there are guardian spirits, those that were loyal to a person in life, so they guard them in death, like the bakeneko or the dog spirit. Since we do not see a lot of guardian spirits around, it is probably safe to assume that they move on to the afterlife after the person they are guarding dies.
Then there are actively malicious spirits. Serial killers, abusers, you name it, corrupted in life, heavily corrupted in death, they latch onto unsuspecting mortals, leeching off of them, being dicks in general, those can only be exorcised by a strong medium or get their asses kicked by a far stronger spirit.