By far the most effective ceremony in the Scrum framework is the retrospective.
The point of Scrum and any other agile practice is to set in motion a constant state of learning, analysis, and synthesis (integrating the learning into the execution model).
By removing the retrospective from the process, the team is deprived of one of the primary learning opportunities that will help them perform better. If the team doesn’t consistently retrospect, then the ability to learn is diminished If the ability to learn is diminished, then the ability to improve is diminished.
Its not just about the iterations
The iterations are a convenient cadence to set up whats really important, which is the retrospective at the end of each cadence. The reason sprints are 1-4 weeks are so that the lessons and learning opportunities are of a good size to actually be able to integrate into the next sprint.
Any company who decides to omit this important practice from Scrum will miss out on one of the most important ceremonies in the framework.