Some herbs do fine with water but basil is so delicate that it tends to turn a dark color and can get freezer burn. It will preserve better in oil, keep its green color and store longer without freezer burn which can sometimes ruin the fresh flavor; plus it will infuse in the oil during storage.
-- Also, when the herbs are stored in oil, they're ready to use for cooking, you can just put them in the pan to thaw and cook.
Some herbs like dill and cilantro, I will freeze the whole fronds/stems. The texture changes but the flavor is just like fresh, unless---freezer burn! I just let it thaw and fine chop it for the flavor, and fresh color.
