The French "victory" at Verdun, such as it was, lay in the fact that the primary German objective was frustrated. Unlike most of the other major battles on the Western Front, the German attack at Verdun was not primarily intended as a way of capturing ground or in the hope of breaking through the trench lines, but to start a battle of attrition: the German attack was conceived from the first - by the German commanders General von Falkenhayn and Crown Prince Wilhelm - as a trap for the French, forcing them to commit vast numbers of men to a barely tenable position so that the fighting force of the French could be steadily ground down. This they certainly managed, but losing as many men themselves was not part of the plan.
In fact it is arguable that the German plan worked in a way: the French losses in the war, of which Verdun accounted for many (although actually fewer than they suffered in the far less renowned Bataille des Frontières, the initial attacks across the German border in 1914) were large enough to affect the birth rate of the smaller French population over the subsequent decade, and manpower shortages were one of the main reasons for the French army's weakness a generation later in 1940 - along with an obsession with preparing static defences of the kind that were so badly handled at Verdun - the Maginot line was the focus of French defence between the wars precisely because Fort Douaumont and Fort Vaux fell so embarrasingly easily in 1916, the one because it was virtually unmanned and the other because nobody had thought to maintain its water supply.
The battle subsequently became iconic for the French, not merely because of its ferocity or casualties, which were mirrored elsewhere, or for its outcome, which was little more than a stalemate, but because its duration and the French army's rotation system meant that a very high proportion of serving French troops - and thus a very high proportion of the male population - served there at one time or another during the year-long battle.