Cleaning a house means more than just dusting and mopping. For a home to be completely clean, the less visible locations need to be dealt with as well, and that means the basement, attic, and crawlspaces.
These locations can hide some of the very things that cleaning is meant to dispel, and not cleaning them means that those forces can fester and use those locations as a staging ground as they invade the rest of the house. With all of the various insects and molds plaguing home-owners, it can only help to clean the house thoroughly and those locations need to be cleaned just as much, if not moreso, than the rest of the house, and companies such as GreenPro Cleaning can help.
While not all homes have a basement or attic, most do have some sort of crawlspace. While the crawlspace may not be as heated as the rest of the house, it may not be as cooled as the rest of the house either. While this means that the crawlspace may be cooler than the rest of the house in the winter months, the area will be warmer during the summer; combine with any kind of humidity, and that creates a great environment for mold.
The mold can then easily spread to the rest of the house through the ducts and other connecting crevices, virtually ensuring a mold problem. It doesn't help that the crawlspace is rarely cleaned, even after the house is built. This means that there is the possibility that garbage from the construction of the house could be lying around.
Dust also builds up in, and all of these factors combine to insure that there will be something for insects to gather. Admittedly they may not be the larger insects, but even dust mites can be a problem. Also, the dust represents a build up of other pollutants that rather than being released to the local environment can build up to a critical mass and cause health problems later on. Between the potential for mold, the collection of insects, and the build up of pollutants, cleaning the crawlspaces is a paramount necessity.