What was so important about Stalingrad in WWII? (why it was so important to both Russian & Nazi armies?)?

it wasnt that important...it was Hitler's biggest mistake (or a tie rather with the mistake of starting Operation Barbarossa so late in the year) to order the 6th army to invade it and try and "Capture it"..as they could still stop traffic up the Volga and should have focused on the oil refineries and staying secure over the winter instead.