When do I need a pressure washer?

A pressure washer is useful for cleaning any exterior surface that has caked-on dirt, grease, plant residue or other material across a large surface area. 

To prevent injury, avoid pressure-washing anything living, whether it's people, animals or plants. If you're pressure-washing near plant beds, use garbage bags or other plastic coverings to keep the spray from damaging your plants.

Since pressure washers are mainly used for larger or dirtier jobs, it also doesn't make a ton of sense to use them on very small areas, unless it's a stain that you just can't get rid of. If it takes longer to pull the machine out and set it up than the amount of time you'll be using it, consider cleaning by hand.