Self closing tags? But why?

Question

There are some tags in HTML that are self-closing..
can anyone tell me why is that so???
(Need appropriate answer):-)

Answer 1

self-closing elements are not in pairs because in most cases of we do not write content in it , as we do in <p>This is content</p> unlike <img>This is content.?
Hope my answer will be satisfying to? A'nt it