The major use of such machines is in connection with a business, involving short-distance deliveries of goods or taxi services, usually in inner-city areas where movements of motor vehicles are restricted either by law or simply by volume of traffic! Beware that workbikes are a bit like recumbents and folders: enthusiasts tend to go a bit OTT, so take their eulogies with a pinch of salt.
For occasional transport of larger loads by a private individual, it has to be said that a cycle trailer generally makes most sense. But if you have a frequent need to carry more than will fit into panniers, you might nevertheless consider making a workbike your daily workhorse. Some of them really aren't much more cumbersome unloaded than a standard bike. They're big and expensive, but if you have the storage space and the money there's no harm in adding one to your existing stable – especially if it lets you get rid of the car!
The easiest and cheapest workbike option is to extend the back of an existing bike to increase the wheelbase and make a kind of long rear carrier onto which you can hang all manner of bags and containers, pannier-style, or even mount a passenger seat.
It makes an interesting d.i.y. welding project, or you can buy a ready-made conversion kit called an Xtracycle. The conversion isn't much more unwieldy than a regular bicycle, goes through the same gaps in traffic (better than most trailers in that regard) and could be considered as an everyday runabout even if you don't need its extra carrying capacity every day.
Purpose-made delivery bicycles begin with the archetypal "butchers bike". The big front basket may not hold much more in total than you can cram into four panniers on a touring bike, but it'll take bigger single items, whilst smaller packets are easier to get at. This kind of bike is often used for internal transport on industrial sites and large numbers are employed by postal services around the world.
Each country seems to have its own favourite design, some with a smaller wheel and bigger basket in front, others with rear panniers too. All incorporate that essential feature of a delivery bicycle: a sturdy propstand. Much bigger loads can be carried on two wheels if they're further apart, with a low-load platform or container inserted either behind the rider like the Burrows 8Freight illustrated above, or in front as per the traditional Danish "Long-John".
For even larger and heavier loads you'll need a tricycle. The simplest arrangement is the traditional "stop me and buy one" ice-cream cart: the back half of a bicycle hinged to a big two-wheeled box. Christiana of Denmark still makes that kind of trike, updated with a steering damper to suppress the main disadvantage of cart-type steering: a tendency to veer off-course whenever one of the front wheels meets an obstruction.
A more sophisticated variation on that theme fixes the box and pivots the front wheels like on a motorcar. That's nicer to steer, at the expense of a narrower box or wider gateways.
Although "tadpoles" (see ourtricycles  page) appear to be more popular, for many purposes it's just as convenient to carry the load atop the wide rear axle of a "delta" format trike. Most pedicabs (pedal-powered taxis, of which Cycles Maximusprovide a good example) are made this way and it follows that's also an excellent way to transport two small children, for example the Pashley Picaback.
Finally we also have load-carrying quadricycles. Indeed the only good reason to add the complexities of four wheels (suspension becomes essential) to a pedal-powered vehicle is because you simply must carry more than can be loaded onto three. Another reason is you want to pedal in a recumbent position.
Generally it makes more sense to sit upright on a workbike: the option of standing on the pedals helps get the load moving and you'll never go fast enough anyway for wind resistance to matter very much. And besides: a recumbent human occupies a lot of potential cargo space! So if that's how you prefer to pedal, a workquad may be the way to go.
For more about workbikes check out the workbike.org website.