Here is an extremely basic description on how this technically works.  And, a list of possible applications which could make further developments economically feasible beyond the cool factor:

Researchers have now built a working tractor beam that uses high-amplitude sound waves to generate an acoustic hologram which can pick up and move small objects.

The technique, published in Nature Communications, could be developed for a wide range of applications, for example a sonic production line could transport delicate objects and assemble them, all without physical contact. On the other hand, a miniature version could grip and transport drug capsules or microsurgical instruments through living tissue.

What other types of applications do you think an actual tractor beam like this could be used for?  Feel free to comment with your ideas.

thanks to esciencenews.com for the great info