Tool cooling. I use a straight cutting oil, soluble is poor unless u use it all the time. My new lathe was badly stained after first use when it was looked at the next day.
that was the motivation for straight oil. Fortunatly it was easily removed. The lathe is now 30 years old and is still shiny and looking over most of the machine like new
always applied with a brush or drip feed from a gallon can, but cnc will be working on its own so washer pump will do

Graham