A wireless site survey pinpoints all the potential problems of installing wireless solutions. Not only are wireless solutions subject to interference between the different wireless access points themselves, but they are also affected by different devices in the environment. These could be high voltage devices such as welding posts, devices that use induction (so most modern electrical engines), devices using the same frequency like wireless alarm systems, etc.
The results of the wireless site survey are used to plan the appropriate number and type of access points to resolve these issues. Moreover, the survey maps out where these access points should be placed to provide optimal coverage.
The upfront investment in a wireless site survey actually saves you time and money in the end because it prevents you from buying more access points than needed and, since the access points are positioned first time right, you don't need extra cables in the ceiling to reposition access points.