Using code for layout has several advantages:

- easier version control and code merging

- you don't need Interface Builder to review the layout code

- code is easier to search, e.g. for the use of certain controls (Xcode doesn't support searching XIB files)

- code can be parameterized (you can e.g. use global constants for font sizes, colors and margins)

- layouts that follows certain rules (fixed heights and margins etc) are sometimes easier to build with code, especially when the number of visible controls is variable or some controls are only optionally displayed

- you can easily refactor aspects of the layout into reusable components (i.e. functions or classes)