The hyphen in self-sealing is used because?

It denotes a single word and not two as it seems to be.

Explanation:

A hyphen (-) is used to show a joined word or more than one words joined together to make a single word. These new formed words have a combined meaning in its usage and will stand for a single word.

So, with the use of the hyphen (-) in "self-sealing" denotes that the word stands for a single word. When used separately, "self" and "sealing" stand for different meanings, self is oneself and sealing means the ability to join together. But when taken together as a single word "self-sealing", it means that the "'object or thing' is capable of sealing itself without the need of any other thing.