Cotton contains a natural oil. In towel manufacture, not all of the oil may have been removed from the cotton before the towels have been made. any oil will left will reduce absorbency, so it's a good idea to give the towels a really hot wash before first use. This will remove any remaining oil that's present and improve absorbency
Conditioner... Debenhams used to have a sign up in their towels department saying not to use fabric conditioner on towels. It makes the fibres slippery which leads to fibre loss in the washing machine and in the tumble dryer. And by putting a plastic film on the fibres, which is what conditioner does, it makes the fibres less absorbent.