irregular opening

why one must  also learn irregular openings?

Noone forces you to learn anything. Some people learn unusual lines simply because they are often fun to analyze. Some people simply like to study openings. Some people like to play bizarre stuff. Someone decides to book up on an irregular line against which he has experienced trouble.

But since offbeat stuff is usually meet well enough by following general opening principles, you can of course avoid studying unusual lines. 

to  asurprise opponent ishalf of its defeat.

fear not where the strong and brave go,this unusual  lines are  not  yet  well scrutinized by games especially to the surprised ones.

Ive played e4 e5 qh5 which is irregular and Ive done it because I like it. I probably wouldnt do this in a tournament or important game but I do it for fun sometimes.