Any pressure washer will wash away dirt, and mold. However, if you have a vast area to clean, a larger industrial machine will clean faster and more efficiently.
For instance: If you had a driveway and a deck area surrounding a pool, you may have a few days work and it would be tedious, should you use a small electric washer or a 5 HP petrol motor driven for that matter.
A 13 HP, 4000 PSI is what I consider an entry grade professional set up. With that pressure, you can support a full size flat surface cleaner.
That same job could be possibly finished, and then you could even move on to a second job in the same day with the right machine.
Use a rotary (turbo) nozzle on the small one and it will increase your pressure alot. Use a turbo on a 4000 and you can remove paint, and stubborn stains.( it can etch into concrete if you work it in 1 spot.) A turbo nozzle gives the power of a 0 degree tip and spins it to a cone pattern giving very effective cleaning results.