Most Modern Oil Paintings that I have worked with are simply too stiff right out of the tube for my particular style of painting. Mediums will help dilute the oil paint and make the paint flow better, which means you will be using less paint. Using less paint, means you will save money.
There are many different kinds of mediums that are used for different reasons, like speeding up the drying time of paint, improving gloss, etc. Do not be overly concerned with mediums right now.
