Since the outdoor lighting fixtures suffer the environmental elements 24 hours a day, 365 days a year, they are pretty abused by nature as well as the humans and critters enjoying the outdoor spaces. They have to deal with things like hard water, pesticides, herbicides, minerals, dirt, plant materials, fertilizer and the list goes on and on. It’s safe to say that they can get pretty messed up over a fairly short period of time.
Remember that professional outdoor lighting fixtures have to be able to withstand to a lot of abuse, so you’re probably not going to hurt them using these cleaning methods. The fixtures should be cleaned at least twice a year to insure that they put out the light necessary to maintain the integrity of the original design — and I know you want your lighting to look great at all times.