What do they do?

Well the obvious answer is they clean. But it's more than that. They are doing the literal dirty work others don't want to do. They mop the floors, clean the bathrooms, handle and dispose of waste and trash. While cleaning may seam simple and menial its very important.

Why is it important? 

It's important because keeping an area clean prevents disease and illness. That's also what janitors and custodians to. Their efforts keep the workplace clean and organized. Which slows down the spread of sickness and disease. Not only does it help you physically, it can also help you mentally. Working in a clean and sanitized environment lowers stress. Also imagine trying to work with trash everywhere, weird smells, and work place hazards all over. 

Janitors and custodians are also easy to find. https://www.ijcsa.org/Janitorial-Service-Directory-Find-A-Janitorial-Service-JanitorialServices is a good link to find Janitorial and custodial businesses near you. You also know you're getting certified companies with references.  Just be sure to treat all janitorial and custodian staff with respect as their job is important too our day to day work life.