Non-modular - all the cables are fixed to the PSU
Modular - some cables are fixed to the PSU, plus a bunch extra you can attach if you want
Fully-Modular - no cables are fixed, attach what you need
It comes down to neatness. If you don't value the neatness of your rig, then save some cash and get a non-modular PSU.
Secondary considerations
- less cables can improve airflow
- cable clutter can cause noise
- cheap low-quality PSUs are usually non-modular