At one point, the Romans extended the frontier in central Germania to swallow up a modest bit of land that was rich in silver mines. IIRC this was around the time of the Emperor Domitian.
Sources: The primary source was from The Grand Strategy of the Roman Empire by Edward Luttwak. It discusses the annexation and mentions the region as securing some fertile land and supporting a friendly tribe, as well as pressing on the hostile Chatti. It also provided a buffer for Maintz and the flank of the annexation of the Agri Decumantes to the south.
However I did find a supporting remark from Tacitus' Annals Book XI, 20, from the time of Claudius:
Nor was it long before the same distinction was gained by Curtius Rufus, who had opened a mine, in search of silver-lodes, in the district of Mattium. The profits were slender and short-lived, but the legions lost heavily in the work of digging out water-courses and constructing underground workings which would have been difficult enough in the open. Worn out by the strain — and also because similar hardships were being endured in a number of provinces — the men drew up a private letter in the name of the armies, begging the emperor, when he thought of entrusting an army to a general, to assign him triumphal honours in advance.
Here Curtius Rufus gets triumphal ornaments by sending his troops over the border to do some quick work Silver Mining in Germany. This is the same area that the lines would later extend out to encompass a generation or so later.