The straps that you are referring to and have not used are called " velcro straps ".... these little handy gadgets are used to assist in tidding up of cables that are just lying around loosely in a case and more so when it can be viewable through a polycarbonate see through side panel.
these straps.... on one side is a carpet like material called "loop"..... and the other side is a plastic material called "hook"..... put them together and you got "hook and loop". why is this important.... well... when these two surfaces come together... they stick to each other and then you can pull them apart with ease.
How is this strap thing good? well.... after you organize the cables inside your case.... you can use these straps to bundle the cables together so as they will help in organizing the vast array of loose cables, help in NOT blocking air flow through your case, prevent accidental pulling of a cable because it was in the way, and Most Of All..... help the visual effect in "neatness" and especially when you have a see through side panel.
The straps that are provided by you psu is not enough... usually you would need to get more of this type or other various type of organizing materials. There are vast assortments of materials that you can use to tiddy up the cables with in a case.... i.e. zip ties, sandwich baggie ties (would not recommend unless you want to be label something not so pleasing.. lol ), cable wraps, cable trows, etc......ones own imagination is the limit. OH... and sometimes you can hide cables without the use of any of the materials or at least using smaller amount of organizing materials.