Two things to keep in mind. Firstly, being that we don’t wear four cornered clothing, besides for a scarf which is Patur, we would never wear Tzitzis. Therefore it would have to be implemented as a rule to get a four cornered clothing so that the Mitzva sould not get forgotten.
Second of all, the Torah says that Tzitzis will remind you to keep the Mitzvos. The Gemara relates how someone was about to sin, and upon seeing his Tzitzis he refrained. This is a reason to wear it all the time.
As for chein’s question of nighttime, it is an Inion from the Arizal to wear Tzitzis when sleeping.