They usually need experiences working on and discussing measurement problems to understand why identical units are necessary. We have observed teams of children happily mixing adult and child shoes of different sizes to measure the length of a classroom. They then confidently reported that the room was a certain number of shoe lengths long. Only then did it occur to the children that they had not used identical units. Through this discovery children came to realize the importance of identical units for accurate measurement.
Most children understand the idea of iteration.
However, they may have trouble simultaneously keeping track of the number of times they have repeated the measure and the place they have left off. Making marks may help children keep track of their place.