Not every bassist wants every string to feel the same. Some prefer slight differences between strings, while others build custom sets for a more balanced feel. Singles are also handy for going heavier on the low string in a drop tuning.
Changing one string's gauge changes the balance of the whole set, so build custom sets thoughtfully. Most beginners are better served by a matched set.
Make sure each string has the right gauge, winding length and taper location for your bass. This matters most for low B strings and extra long or multi-scale basses.