Because if you can be claimed as a dependent for someone else it may change your tax return. I claim my son who is 25 yrs old but somewhat handicapped (not disabled) so he will live with me most of his adult life so I claim him as a dependent however, he does work and file his own taxes so he must check the box that states he is claimed as dependent on someone elses taxes. it doesn't slow down his tax return at all and it keeps everything accurate.
