Sometimes bigger is better, but why do we think this? Recently, I discovered my local gaming group is to host a store-vs-store 40k apocalypse match (for non-gamers, a very very big game). Whether or not I attend, I have been inspired to purchase a Malcador Infernus. but, having thought about it, I have asked myself "what is the super heavy's purpose?" Thus, I have devised a number of reasons you, as gamers, should invest in the right super heavy for you, with examples given for each way of usage.
The 'Bunker' Super-Heavy is essentially a huge rolling building, its sole purpose is to protect its occupants, so that they can hold the ground, or transport them where need be in relative safety. The Crassus and Gorgon both suit this, the first is faster, whilst the latter is more protected against shots.
The Fire Magnet
'Fire Magnet' may sound a worrying term, but in a way, it is a sound strategy. With this idea, you choose something intimidating, scary and large, thus worrying opponents and forcing them to take out the 'threat' that is your Super Heavy, whilst your plan was to use it as a distraction from your other, more important units. Anything with good front armour or a big looking gun is an ideal candidate, so pretty much anything baneblade or bigger is suitable. Squiggoths and heirophants work wonders too.
(Images from Forgeworld.com)