Consider:
She'll listen to music, alone in her room, for hours.
Sulphuric acid will dissolve most metals.
This is an example from OALD. The 8th definition for will is used for talking about habits.
And another example is offered, which is from Practical English Usage.
But if I said
She often listens to music, alone in her room, for hours.
Sulphuric acid dissolves most metals.
Would this bring any difference?
In what situation is will used in this sense?
In short, both versions of your examples mean roughly the same thing. The version expressed with will is used when the speaker expresses an idea as an opinion. The other version, expressed in the simple present tense, is used when the speaker wants to express the idea as a truth.
Technically, this is epistemic modality vs. apodictic proposition, or, to put it simply, opinion vs. fact. According to the page Alethic modality,
"The criticism states that there is no real difference between "the truth in the world" (alethic) and "the truth in an individual's mind" (epistemic)."
Here are two examples from Wikipedia's Modal verb, showing a typical simple future (deontic modality) in contrast with an expression of speaker's confident opinion (epistemic modality) readily:
She will try to lie. -- epistemic modality
I will meet you later. -- deontic modality
OALD's definition, "used for talking about habits", means exactly what it says. This is closely related to other senses in the OALD's definition. (Your sulphuric acid example is more of sense 6, "used for stating what is generally true". Also, the statement "Sulphuric acid dissolves most metals," can be considered as an apodictic statement.) This is because will is a modal verb, and one of its main usages is epistemic modality, which, as the Wikipedia page states, "deals with a speaker's evaluation/judgment of, degree of confidence in, or belief of the knowledge upon which a proposition is based."
Will is used when the degree of confidence is really high, and one reason that can make the confidence such high is because the habituality can be observed.
I think this applies to continuous and progressive aspects.
Thinking of the sentence, "She'll listen to music, alone in her room, for hours." the word "will" describes an indeterminate state. She could be now, might be in the future, or often is found, or has a history of being. This is said to be continuous - You are defining the actions of a person who is in a particular (albeit unknown in this case) state.
Whereas "She often listens to music, alone in her room, for hours." defines a specific state, and is said to be more progressive - you are defining the state of the person performing the action.
To put it in another perspective: If you were telling a doctor about an ill person, you might say "She will listen to music, alone in her room, for hours" because it is an action that might be related to her current state.
But if you were describing a friend, you would say "She often listens to music, alone in her room, for hours" to establish the state she is in.
In most everyday English, the latter would be used and is interchangeable with the former.
For more on continuous and progressive aspects see http://en.wikipedia.org/wiki/Continuous_and_progressive_aspects
