Why do some semi tractors have twin steer axles?

Tandem steer axles are primarily employed in vehicles because of that particular vehicle's load distribution, per-axle weight limits and bridge formula issues. They're often used for crane applications, large cement mixers, dump trucks, and for some oversize load hauling applications, etc. -- vehicles where a single steer axle would be inadequate or undesirable. Single axle load limits are typically 20K pounds, tandem load limits are typically 34K lbs. How both the payload weight is distributed and how the axles are distributed can have a substantial effect of how much weight a particular axle has to carry, and to remain legal additional axles are required -- however just putting extra axles around the rear drive axles will do little to ease a heavy forward located load. Also, with tandem steer axles the tires are actively turned, unlike tag axles which tend to passively turn/align themselves (or not turn at all). By having a couple active steer axles one can gain a more positive steering response (not to be misinterpreted as meaning a tighter turn) which can be particularly useful in off-road applications, and by having the forward (steering) axles actively aligned in turns rather than having an extra forward placed axle just passively tagging along (sorry, had to) less tire scrubbing should result.