A street that is often asked about whether it is optional or not in Wizard City is Colossus Boulevard. Well is it? It's not required but you should do that street. But what is the point of working on it if it's not required? Not only do you get extra experience, it is a practice for Krokotopia. In the next few worlds, you face two monsters at once. No, not in a tower or against a boss, actually in a area. Colossus Boulevard does that too, and the point of that street is to prepare you for future battles. Well, I hoped that helped.
1. It's just a fun little place anyway. 2. There is a secret shop and a dungeon for level 50 players. 3. If you are Ice (and I think Balance goes here once too) you come to this location to learn some of your spells. 4. You must at least be able to access it if you want to start Daily Assignments (you must go here to enter the Order).
Grizzleheim is much the same way, it is not required but has some fun stuff and you can learn 2 new spells via sidequests in Grizzleheim/Wintertusk. Wysteria is also fun side world that you can complete.