There was a whole ton of these, only took one, they were all in a giant crate, whats this device do? how do i use/test? is it worth anything? should i grab the others? they've actually been sitting in the same crate ever since i've started snooping about, never thought to grab one until now.

more pictures

Those are used in hydraulic and pneumatic cylinder systems to indicate the start or end of stoke of the piston, they use inductive sensors so you know when to stop applying power to the cylinder, those are very expensive little buggers, new like that one looks, they are worth about $900 each, but used they are only worth about 10% of new.

They are something you would find in automated machines used in manufacturing using PLC controllers allowing things basically to run un-supervised by humans..   

why is such a small part so expensive? also why would a scrapyard have a huge crate of such an expensive item just laying out in the elements? do they not know?? 

It's one of those things if you need one, and only one or two places make it you pay the price they demand, and they really are pretty specialized items so they are not really common place and the manufacturer knows most factories can't afford downtime some large places that can add up to millions of dollars a day so they also know they can get demand the payments needed it's like holding someone hostage.. That and everything in the world of hydraulic's are expensive, and they are rated to 3000 PSI so there is also some engineering going into them, and unlike limit type mechanical switches they are very accurate and since they are also mounted to the cylinder they don't get in the way and for the most part are highly unlikely to be damaged..

went back again tonight, did not feel comfortable taking something so valuable, left them in the crate, however did retrieve a new ballast and a few other little bits and pieces

Where i work, we trash everything is not ok, if there were a lot of them, all they have failed once or, in best scenario, they weren't needed anymore (obsolete/unused part after some mechanical upgrade).