For as long as humans have been carrying things with them, we've also been misplacing and losing them.

These items will sometimes make their way into the ground and stay there unseen even for thousands of years depending on the material and location eventually being dug up either accidentally or by metal detectorists and archeologists.
 
Metal detectorists and amateur archeologists contribute with thousands of significant finds each year, these items would have otherwise been further damaged by natural as well as farming and even building on top of them.

With the use of metal detectors we can find metallic items in the ground and hopefully build up knowledge of history locations piece by piece.
 
Relic hunters vs Treasure hunters

Relic "hunters" such as myself are happy to dig up all the trash and iron in the field if they can find a couple of historic bits such as some ww2 bullet casings and a crotal bell. Where as " treasure hunters " only dig the tones likelier to be something of value such as a gold coin or silver broach etc.

And no, you can't just set it to only find gold, you have to learn your machine to know what to expect and no you won't get rich, infact finding something of value is extremely rare for relic hunters such as myself who love history over value mainly because we spend time digging rusty nails.

Point made by one of my favourite finds being a Royal Artillery button dating around 1790-1802.. who doesn't love cannons?