The principle of fortification has not altered, which is to deny access or progress through a given area. Fortifications, particularly prepared sites like Verdun are cost and labour intensive at the front end, but if done well, during war it can be quite economical in that far fewer men can command the ground. The main problem is that these works are static, and rely upon the enemy choosing to confront them. So much easier to find another avenue of approach, which is the gist of the Schlieffen Plan, the German button-hole to the right, through Belgium and into France, thus avoiding the difficult terrain and extant fortifications along the French/German frontier.

Now, here's a lesson in itself: While Germany had no bones about forcing their way through Belgium, they did ask nicely at first. Realistically, all Germany had to do was contain Belgian forces while maintaining Their advancing drive into France. They want Paris in six weeks, so they haven't come to subdue Belgium at the cost of time. The Belgian forts were invested but whether or not they fell was immaterial, the smart idea is to bypass impassable objects whenever practicable.

These mass fortifications offer little more than a deterrent for advancing on a particular axis, and comfort to vulnerable populations along contested borders. So the question then persists as to why choose to assault Verdun. It's a dangerous prospect. The firing positions OP mentions are mutually supporting; meaning that the fields of fire for the forward position interlock, concentrating shells and bullets at places deliberately built to channel an attacking force. The entire chain of fortifications is covered by artillery in depth to the rear, well wired along the front. The purpose was not, ostensibly, to take Verdun. Putting pressure on it was hoped to (and did) draw more and more French units from elsewhere on the Western Front in order to prevent its fall or to re-take portions in German hands. German Chief of General Staff, von Falkenhayn wanted to "bleed the French white," his goal was to tie up and eliminate French manpower in a battle he estimated the French couldn't bear to lose, Verdun having a revered history to France.

Doing this was projected to accomplish several things. First was to weaken the Western Front at large by forcing the French to concentrate in one area. This then prevents, delays, or what wound up happening, pushing forward Allied timetables for their own offensives. Haig had wanted to attack in 1916 closer to Ypres than the Somme, and had to compromise his preference because the French were under pressure at Verdun. He also had wanted to begin his campaign in August, but went ahead a month earlier in deference to the French, and was positioned to attack on a wider front than he had initially envisioned for the length of the line the British held by summer 1916 included positions formerly maned by French troops who had been moved to support Verdun. Some of these items were part or cause of the initial failures of the British attack at the Somme. (see: Holmes, Richard "The Western Front".)

The defensive phase of operations is properly a means to an end. The idea is to position one's forces in such a way as to be able to repulse the enemy long enough to bring overwhelming force against the enemy. It is a play against time, and is enacted on all scales of organisation from small patrols covering each other to entire armies arrayed in a network of entrenchments and fortifications. Breaking through is always best done via the path of least resistance, no one would invest a fortification if it could be gotten 'round. Falkenhayn was creating a path of least resistance by tying up so much French manpower in one area. He didn't need to capture Verdun, and it worked better for him the longer the French resisted. Plus, there was always a chance (and this remains speculative), that the French would drive themselves to destruction and sue for peace, with the knock on effect of having the British evacuate the continent. In this case, we wind up not having history repeat itself, but work in the direction of original intent. The Battle of France in 1940 demonstrated that by maneuver and speed, frontier fortifications could be bypassed entirely and the enemy engaged in the open.