I'm sure that your source of irrigation water was tested when the nursery was developed but water quality does change. Surface water sources can easily become contaminated from runoff and even very deep aquifers that were once considered to be pristine have been found to be contaminated by nitrates or pesticide residues. Therefore, more and more nurseries are installing some sort of water treatment system to protect their crops. Be wary of vendor claims and talk to water quality specialists and other nurseries before investing in a system. All systems have limitations and life expectancies, and require routine maintenance, monitoring, or both.
Distillation is slow and consumes considerable energy, and is therefore relatively expensive.
Distilled water can corrode some metals such as iron and copper. Filters must be replaced regularly or contaminants will be released back into water. Poorly maintained filters can be breeding grounds for bacteria. Inorganic minerals, especially salts such as sodium, calcium, magnesium, boron etc. Also effective on volatile organics and some pesticides.