For the website itself, it's does absolutely nothing.

But when you share the website, most devices these days will display a website preview (a picture), a message, and a title.

If you DON'T customize this, the device will decide what picture from your site is displayed and in what message.

If it can't find a picture, it might just choose to display a cog and wrench symbol, which looks TERRIBLE.

If you've created the most eye catching site ever seen by man, but upon being shared, it displays a random preview with no message and a picture that doesn't make any sense in light of your brand or even worse a symbol that makes your site appear to still be under construction, it can seriously damage the amount of people who will actually click to view your amazing site.

Basically, it's for advertising.