In that instance I can think of perhaps the economical uses of wood, for furniture and for charcoal which the Romans were using for industry and additionally for warfare also (siege engines in particular), especially when one considers that in other theatres of the empire wood was relatively scarce. Wood from the forests of northern Europe would likely command a high price in the scorched lands of Egypt and Assyria - a price that could be paid in gold and silver.

In the ancient world, material wealth was extremely important. Unfortunately as the Empire grew and absorbed the demands of other cultures it saw opportunities for economic growth that might be considered unsustainable. Potentially this is why they pressed into Germania inferior to begin with.

They went in equipped for war expecting a risk-reward ratio that would match their profit margins. The risks however were far, faaaar greater than they reckoned with.

In addendum; after having done a bit of research, Julius himself had a couple of run ins with scorched earth warfare tactics. The Persians also were the recipients and perpetrators of the tactic at various times, as were the Helvetti Celts.

Perhaps the reticence of later Roman figures to commit to this kind of tactic is indicative of a changing mentality on behalf of legionary generals.