I'm playing on ps4, so can only have 16 icons on the bar at once, so once I've included my main skills and abilities that I need for my rotation, it means I need to use the expanded bar a LOT for the debuffs/procs/defensive skills. I've noticed that quite often I'll mess up on a rotation because while I've been switching to the expanded bar, I've missed a proc and have to restart.

By using macros to chain things like swiftcast > flare, I can condense 2 or more skills into one slot, reducing the number of button pushes and therefore minimising the time needed for each rotation. It may only be a few fractions of a second, but they add up!