Pochita is the Puberty Devil. The manga is about Coming of Age.
\-Through out the whole story, Denji never seems to be able to "character develop" or "stop thinking with his dick" but the problem is no matter how much you know and how smart or experienced you are, you cant character develop out of hormones and the desires that come with them. Denji breaks down in the train part because he doesnt understand this so just like the fans he is mad/dissapointed in himself.
\-Pochita coming out at Denji's lowest points represents the destructive and uncontrolable meltdowns that come with unstableness of puberty.
\-Around the time they fuse matches up with the beginning of puberty.
\-Denji getting chainsaw powers represents the adult responsibilities and the hardships that come with it. He now has a duty to protect those around him.
\-Because he is now powerful/capable, there are predators around him wanting to harness his usefulness for their bidding, more on the next point.
\-Makima represents the workforce and society, using his desires and needs like food, sex, socialization and self actualization to manipulate him to serve the government's needs.
\-His dynamic with Power that starts off as something as pure sexual and beneficial, later in the post darkness devil arc leans more into the concept of putting aside your selfish desires to protect those that need you. In an earlier point if they were naked in the bath together, Denji would have felt incredibly horny but by that point he is putting his adultish responsibility of "protecting someone who is in utter terror and need of help" above his irresponsible sex drive.
\-Eating things to make them disappear from life represent the ability to decide with what/who and how you live your life. An adult may chose to cut someone or something out of their lives with their decisions like moving away after which they can 100% pretend that thing never existed. also joke about being more hungry overall.
\-In the final chapter, Denji is so void of everything because he doesnt have any of those things that would drive him.
**More about the ending and how it all ties in with the concept of coming of age:**
Fujimoto showed the moments which would have been the monumental to somebody's life except there aren't any hardships anymore so all of it feels pointless.
Many fans wished that Makima never existed so that she could never abuse Denji but then he wouldn't have gained the things he did by breaking free from her chains.
If power never died, she could not have served as a catalyzer to his wake up. You only act to stop the pain after it becomes severe and for a pain to become severe you have to get hurt severely. That wake up was also really important in his development as he learned how he needs to be his independent person which we saw through out the entirity of part 2 with him breaking out of the 2 binary choices
Many wished for Nayuta's survival but for that she would have to be in a more secure position where she wasn't Denji's responsibility but taking care of her contributed so much to his development even though he failed due to his inexperience and teenage angst/hardship/hormones/dificulty of acting properly during confusion of puberty. We can see that in his emotional distress when he comes to his senses after moments of uncontrollable meltdown.
Fujimoto is trying to show you even though the bad things in your life might have hurt you a lot, you are now a lot better because you got through them by showing you Denji's dogshit boring life without the detrimental things he lived.
You had to see someone you befriended and sympathized with end up stripped of their individuality and with out any of the things they fought so hard for and stranger to most of the people they loved growing up, you wouldn't want that happening to you after going through so many troubles of coming of age. That's why the ending felt so dogshit. Because after all, HE **JUST LIKE YOU** FR! FREE **YOUR** BOY!