Think about it. All of the other available vehicles - online, direct mail, radio, newspaper, cable TV (do I dare still mention yellow pages?) require a significant financial investment. Promotional products, on the other hand, do not.
Say you give a prospect something as basic as a business card size magnet. For a $100 investment you can buy 500 of these promoting your brand and marketing message. You go to a networking event, meet someone you want to connect with, hand them a magnet, and that magnet goes up on your contact's refrigerator or office file cabinet shortly thereafter.
And you can apply the same concept to a pen, can coolie, jar grip and lots of other promotional items that cost little and deliver lots and lots of 'views' in return. And consider this, unlike other forms of advertising, there's very little waste because you decide who gets the promo item and who does not.
No other advertising medium can do this period, let alone do it for the low cost of a basic promotional product campaign built around one of these items.
That's why I often refer to promotional products as 'Entry Level Advertising' for the small business owner. If you're in business, you need to market and no medium allows you to effectively do it for as little up front cost as promotional products