A dirty can can be a problem in more than one way: here are a few of them to consider.
1.) That "Dirty Can Aroma" is naturally repulsive.
This issue is familiar to almost anyone. Since the sense of smell has one of the strongest impacts on us, no wonder the request of "can you take out the trash?" is met with such disdain. It's not necessarily the labor involved, it's our natural strong reaction to the smell you will likely encounter, and our subconscious desire to avoid it. And for good reason: while the smell in itself may not be harmful, concentrated quantities of the gaseous component of decaying items can cause eye irritation, headaches, and shortness of breath. Keep your cans in the garage? The effect just got magnified...
2.) This smell can make other areas repulsive too.
Noone wants to be exposed to the nasty smells that go with your garbage can. No matter where you keep it, a dirty can is not something you or your guests want to be around or have to pass by. This leads to storing them in a remote location, adding to the effort required to take out the trash. And if your HOA requires you keep you cans in your garage, they can quickly create an environment that is unpleasant at a minimum, and possibly dangerous depending on what is growing inside.
3.) Think dirty cans have no friends? Think again....
While a nasty can may be repulsive to people, other creatures are attracted. Flies are drawn to lay eggs which become maggots. Roaches and rodents are drawn in to feed. If stored outside, other creatures may be attracted an prone to investigate, leaving a disgusting mess for you to deal with, and potentially exposing you to other diseases and health risks.
4.) Bugs, Rodents, and Critters aren't the only dangers:
There a number of dangerous bacteria that can join the fun in a dirty can. Salmonella, E. Coli, and Listeria are commonly found in residential garbage cans and dumpsters. These pathogens can make anyone ill, and may pose additional risk for those with compromised immune systems.
5.) A dirty can can get you in trouble with the neighbors, or worse.
Because of the problems they can cause your dirty can may be indirectly causing issues for others. It may be that your storage area is on the other side of the fence from your neighbor's patio, or the rodents it is attracting are putting them at risk too. More and more municipalities are holding residents accountable for maintenance of their waste containers. Some communities and HOA's will actually fine residents if their can is too dirty.
A commercial can cleaning service addresses these issues.
Cleaning the trash can is not a job anyone wants to do! Companies with specialized equipment deal with these problems at a reasonable cost, using very hot water at high pressure to sanitize and deodorize cans so their customers don't have to worry about the problems noted above. They also do it with a fraction of the water that a typical home cleaning would use, and all of that nasty waste water is captured for proper disposal, not left behind on your driveway or lawn to continue "making friends" you'd rather not have around.