I think the Germanic tribes saw Caesar’s bridge over the Rhine as a threat for several reasons. The shock-and-awe of having to watch this seemingly impossible marvel of engineering being steadily constructed ever closer, and across a barrier the Germans likely considered to be unsurmountable, was doubtless the psychological effect Caesar was looking for. By being able to accomplish such a feat, the Germans may have considered that the Romans had the favor of the gods, and thus refused to engage them in battle after their crossing. Another thought is the Germans may have considered the Romans to be a highly-advanced society to have been able to put the bridge up as quickly and efficiency as they had.
Speaking of Roman ingenuity, Caeser would have needed to have a number of items in play in order for his bridge to be constructed so rapidly. The mathematical and engineering knowledge required to build such a structure across the span, and stable enough to support its own weight plus that of his men, and against the river’s current, was certainly necessary. He would also have needed the workforce logistics to cut, gather, and move the lumber and other materials the bridge required. This may have been soldiers, or perhaps other servants or staff attached to the legions. Also, assuming such a large group was encamped in the same spot over the course of the project, camp logistics would have been another consideration to accommodate the daily needs of so many people via foraging, hunting, or other means. Perhaps Caesar had knowledge of the area, knowing a suitable spot, more shallow or narrow, over which the bridge could be ideally built.