If you're asking whether it's compulsory or not, No.

For me, it's more for protection, and secondly, it works great to reduce sun flare when shooting outdoors.

You can think of it this way. A cheap filter costs like RM80, if it's got scratched, you can spend a little extra money to buy one.

If your RM2k lens itself get scratched, chances are, it may be not repairable, and there goes your luxury lens becoming a broken paperweight.

i've attend to many class before.. most of them is basic, after that i start to learn and study from internet.. yes its better!! dont waste your money for basic class while u can get more for FREE from internet!!

look FILTER EFFECT for detail about filter, and maybe your sifu dont know about this also.. 

I assume we are talking about UV filters only. These days it might not be necessary. In the old times, films are more sensitive to UV light, which cause haziness to be seen in prints. These days, digital sensors and newer films are less sensitive to UV light, so there is no need for one unless it's a very hazy day.

So in essence, yes, it's only for protection.