NASA currently developing a technology to collect atmospheric and gas sample using laser coralling. It's not powerful enough yet to tractor big object like a tennis ball, but in the future, who knows?

http://www.nasa.gov/topics/technology/feat...actor-beam.html

Star Trek ideas in science, maybe we will see a working tracktor beam soon...

Function of the tractor beams for what?

To grab and pull things into your spaceship.

Originally, they intended to use it to tractor a space debris in orbit into the atmosphere where it'll burn. But it's not powerful enough. But it can be use to tractor gas and dust particles. This is useful for a deep space probe that visit other planetary body. The probe can stay in orbit and tractor atmospheric sample into it's sample collector and then return the sample to earth for analysis.