The majority of people who vape, do so because they were once smokers and vape because it is a harm reduction method to take in the chemical they are addicted to --- nicotine.
Basically, these former smokers are looking for ways to best replicate the taste and feel of smoking. Nicotine plays an important part in replicating that smoking experience. However, if you never smoked before and are not addicted to nicotine, then why vape with e-liquids that contain nicotine?
Nicotine has more or less a peppery taste which provides what's called the "throat hit", that kick in the back of the throat one gets while smoking or vaping. Therefore, in most cases a liquid will taste a bit different with and without nicotine present while vaping.