As David told you, "will" expresses willfulness. We can also speak about persistence or even obstinacy. While the present simple merely expresses a habit, the future simple (accompanied by an adverbial to denote habit) expresses a willful, persistent, obstinate practice, for example:
- Bobby cries whenever his parents refuse to buy him a toy.
- Bobby will cry whenever his parents refuse to buy him a toy. (Being future, there is also something of a predictable behavior in the use of this verb form.)