An argument can be made to not start promoting a product until after it's available. So, no pre-launch promotion is, IMHO, just a marketing decision.

Starting an online, social media marketing campaign (FB, Twitter, Instagram, websites, blogs, message boards, etc.) is a great way for companies to start small and build as they gain momentum and sales.

It also allows them to work out their manufacturing, distribution, and marketing on a small scale and adjust with the growth of the company.

It's also a cost efficient path to market instead of spending a large amount of money on traditional, more costly marketing methods like TV, paid print ads, etc. It's organic growth that is efficient, manageable, and poised for adjusting to the scale of its sales as it grows without overspending on marketing.

Again, it's one way for a start up to go about introducing its products to market.