Havent watch but why moving cities?

It's just for gimmick. Cause nobody else made a story about moving cities, so it would look new, unique and original. Which was true to some extent.

The meat of the story itself basically just random Star Wars episode. So if they go without the moving cities angle, they would have nothing. It would be just a wannabe Star Wars.

Initially I also thought it was stupid, but after watching the movie it's not that bad, really. Still stupid, but not that bad.

They only touch on the topic in the film, not read the book. The idea is that it is 1000 years in to the future and they ended up in cities movable cities to harvest other cities, which caused other cities and towns to become mobile as well.

I haven't read the book, so I can't say for certain, but the idea of a moving city reminds me a lot of the video game Final Fantasy VIII. In FFVIII, Balamb garden (which is kind of like a military school) floats out of its foundations and moves across the land in order to avoid an enemy missile attack that was targeting its location.

Considering that the story in Mortal Engines is set in a post-apocalyptic future, and the fact that the story line involves the discovery of an ancient weapon, I'd say it's likely the moving cities were a method societies used to avoid nuclear attacks from their enemies. If the rest of the major cities were taken out by nuclear attacks, it would be the mobile cities that would stand a chance of avoiding such an attack.

It also reminds me of Metal Gear Solid, where Metal Gear was designed as a mobile nuclear launch platform that would negate the concept of Mutually Assured Destruction. Furthermore, it also has elements of Mad Max, where it is the most mobile who are able to avoid attacks from scavengers that roam the wasteland.