Reasons for dropping subjects

Hi, O level student here. I was just wondering about why people would choose not to turn up for subjects. I really don't see any plausible reason not to show up.

Even if i expect my results not to be good, or i want extra time to focus on other subjects, it doesn't seem practical to forgo subjects in exchange for a few hours to compensate for other subjects.

I'm just trying to understand the stance some of my friends have since they can't seem to give me a straight answer. Thanks!

Edit: Thanks for the responses guys! Was really trying to see it from their point of view and if they had good reasons i just couldn't see. As far as I'm concerned i think I'll take my chances with attempting all the papers.