Its a form of grooming or possibly because it has a itch there. may be irritation such as an itch or a slight pain perhaps

poison ivy

i have the exact same thing it is around my armpits mostly and a little across my chest i thought it might be slight stretch marks

Camphor produces a similar feeling of cooling to menthol, and is easily absorbed in the skin. It's a slight anesthetic, and as such is often used as an anti-itch gel.

An itch is a very slight form of pain from your nerves. There is usually something touching you and your body feels threatened by the contact, and so you itch to remove it from your skin.

In bites and stings it's because the nerves are stimulated by the poison/acid/alkali released into the bite.

Sometimes you may feel a 'Phantom' or travelling itch, when you can't quite tell where you need to itch. This is caused by random stimulation of the nerve, the cause of which is unknown