As stated, a modular PS allows you to use only the cables you need, keeping cable clutter to a minimum. Additionally for many models you can get cables of different lengths from either the manufacturer or a third party for specialty builds.
In a Corsair Obsidian 250D, or any mITX case that accepts a standard ATX power supply, the cables are much longer than required. Alternately, a super tower monster could easily make you want for longer cables.