Certain modes are only available in ranked, so if you want to play anything other than Turf War, gotta play ranked. You can also earn money and experience faster in ranked, provided you win a decent amount.
Splatfest is a 24 hour event where you side with a team and fight in Turf Wars for your team. There are unique Splatfest ranks that determine what your prize will be at the end of it, and you earn those by just playing enough during Splatfest. The prize for winning Splatfest is sea snails, which you can use to add slots to gear or reroll the sub abilities on gear. You can do this without snails, but it costs 30,000 a pop, so the prize is essentially a lot of money. The losing team gets snails too, just less of them.
Also Splatfest has some unique effects:
-the stages are at night
-everyone in the plaza dances
-unique music
-the ink glows slightly
-there are three stages in the rotation and the whole 24 hours is one rotation