Root canal treatment has a bad reputation, and part of that comes from a misunderstanding: many people believe it ends when they leave the appointment.
What is done
The damaged pulp tissue is removed from inside the tooth, the canals are disinfected and then sealed. The purpose is to preserve the natural tooth rather than extract it.
Why the restoration is part of the treatment
A well-treated canal that is poorly sealed from above becomes contaminated again. The crown or restoration placed afterwards is not a cosmetic extra: it protects the work done.
That is why it should be completed within the time indicated, and why you should not bite hard on that tooth in the meantime.
If symptoms return
Retreatment exists: the previous material is removed, the canal is disinfected again and resealed. Where that is not viable, periapical surgery is considered.