You should future-proof your system against future upgrades. If you have a modular power supply, you can add cables as needed or remove those you do not need, thus freeing up valuable space inside your case. If you want the flexibility of adding other peripherals later or want a beefier graphics card, this is ideal.
However, if only a few extra cables are lying around you can get by just fine with only semi-modular or fixed cable systems. Most PC cases let you tuck these away in the back in order to hide them from view.
A word of caution: compatibility is an important factor when it comes to power supplies. Keep cables from the same PSU, as using cables from different PSUs could damage your computer. Moreover, even identical cables from the same manufacturer may not be universally compatible. Make sure that your cable set matches your PSU's PIN connectors if you are going to splinter from the cables included.