From OpenStreetMap Wiki
|tower:type = observation|
|Group: Man made|
|Used on these elements|
|Status: In use|
|Tools for this tag|
|This tag page is a stub. You can help OpenStreetMap by expanding it.
Similar pages for structural reference: Tag:railway=tram, Tag:waterway=dock.
One use of an observation tower is a tower that used to watch for and report forest fire (Tour de surveillance des feux de forêt(fr))