They wanted control of Stalingrad. It couldn't just be ignored like forts in France and mopped up later. Stalingrad was a major industrial centre and it played a major role in the Soviet supply system distributing supplies to and from the Caucasus. The city also had a significance because of its name that if it fell would cause a huge moral loss in the Soviet Union and probably demoralize the high command in the Soviet Union. Also, taking Stalingrad would open up the Caucuses and the important oil fields there to German expansion. Bypassing Stalingrad would have left a major threat behind the German lines and would have been a strategic nightmare.

Sources: Third Reich at War by: Richard Evans

The Second World War by: John Keegan

Stalingrad was considered by both sides to be an absolutely essential objective in the war for Russian soil.

Stalingrad and Leningrad (and a few others, of course) were the "totems" of Communist Russia's ideology and national pride. They represented much to the Russians and were used by the Soviets as a fulcrum for the war effort.

Stalingrad's location and purpose was also a huge factor. It was a trade and transport hub and as a large industrial town, it was notable producer of armour for the Russians (many unusual Russian prototype vehicles were produced in and often fought in Stalingrad).

Losing Stalingrad would have crippled the Soviet morale, and a major supply of armour to boot. The Germans would have had the morale boost needed to push on in harsh, draining conditions whilst also gaining a heavily industrialised city to boot, which was critical considering the Germans struggled to maintain a logistical advantage throughout Barbarossa and those Russian winters in particular.

It's not unreasonable to suggest that Stalingrad was absolutely the do-or-die objective for both sides and if Germany had taken it, the outcome of the war may well have been very, very different for everyone involved.