The biggest thing I learned that saved me much heartache was the difference between crowns gear and school specific gear. Granted, it took me until level 50 when I got my Malistaire robe to figure it out, but from there it was onwards and upwards. Let me explain.
"But look at the spiffy card!" you might be saying. Well, it certainly is spiffy and I'd love to try it out, but it's 5 pips and a Balance spell, so I'm probably never going to use it. Five pips is a lot when you consider that when you're 60+ they are mostly power pips. So that means if I cast Obsidian Colossus with 5 power pips I could've casted...
I think when you illustrate it like that you can see why using spells from your own school is smart. This is another thing that took me forEVER to learn. But I'll talk about that at length some other day.
So in conclusion, the reason it's good to wear clothes that are tailored to your school is because they meet your school's needs. I don't need a ton of accuracy as a myth wizard, but my storm wizard does! Her clothes are suited to meet her needs. Crowns clothes however, aren't tailored to meet any one school's specific needs and therefore can be less beneficial to use. You might have cool new cards and a snazzy matching outfit, but it doesn't matter if you die all the time in battles!
And that's something I've learned.