Recently, I went to open a sample bottle of toner (from a fairly well known brand) when I noticed something dark on the side. I thought it might be a smudge on the outside, but it was in the bottle. It was a dark flake of something, probably mold. I definitely wasn't going to use that toner. This was a sealed bottle, stored in a dark, cool, dry spot. Looking at the ingredients, there were no obvious preservatives.
Preservatives come in different forms (parabens are one type that many have heard of and are trying to avoid) and protect aqueous products from growing mold, bacteria, and other nasty things. Products such as scrubs that don't contain water, but are likely to end up getting water in them, might also contain preservatives. Generally, only a small amount of the preservative is needed- you will usually see them at the bottom of the ingredients list.
I sidestep the preservative issue by not making products that use water. This way I can keep Ivre de Fleurs preservative free, while not worrying about if my product is safe. I add antioxidants, Vitamin E and Rosemary Oleoresin Extract, to many of my products to increase their shelf life. Oils might not turn moldy on their own, but they can go rancid, and the antioxidants keep the oils fresher for longer.
I know a lot of people don't care if their products have preservatives, but others don't want to use products with any preservatives in them. I try to minimize my use of products with preservatives, since I don't know what the long term ramifications of using them will be. I don't avoid them completely, though. If I'm using a lotion, I'm fine with the small amount of preservatives in it, since I don't want to deal with the more immediate issues of mold and bacteria.