Why focus the Flaw?

I love stamps, they are a thing of beauty. The Miniature sheets are much more beautiful and could be literally framed in the walls. I’ve been collecting Indian Stamps, b’coz, its easy to collect since I’m an Indian (living in India) and want to know more about India.
What I find very strange as I browse through sites dedicated to Indian Stamps, the focus is more towards finding defect in a stamp! These collectors take all the pains to identify a defect and say, “Hey! I found this flaw. I’ve these flaw stamps. Wanna buy?”
Why these ppl are focusing on defects and why they want to keep the price of a defect piece more than a correct piece!
I dono if I will trade my stamps but if at all I do I will say, “I’ve this beautiful stamps and they are absolutely perfect. Do you want them?”