The thread title gave me a start. No way someone else here is straying into the rareified airs of linguistic theory and semiotic theory. Way? Please? KInd of. Grammatical mood overlaps several areas of emotional and ambience moods. Linguistics: Language: Voice: Mood and Attitude. Emotion. Grammatical mood signals by syntactical verbal inflection an atttitude toward a subject, be it emotional or not, which not emotional is an emotion too. In combination with aural intonation or inflection and body language, both moods apply to spoken expression. Written word moods can carry a lot of the aural and visual freight of speech moods, that are otherwise lost in translation, when they report interpretable intonation and body language.
Mehrabian's communication studies conclude, for conversation, "7% happens in spoken words [the words themselves], 38% happens through voice tone, 55% happens via general body language." "It's not just words: a lot [of] communication comes through non-verbal communication. Without seeing and hearing non-verbals, it is easier to misunderstand the words. When we are unsure about words and when we trust the other person less, we pay more attention to the non-verbals of what we hear and see."  Though there's a lot of controversy and dissent about Mehrabian's methods and conclusions, they do illustrate why written word communication is more challenging expression than everyday conversation.
Written word expression must make up the deficits. It is very easy to miscommunicate and be misunderstood through writing. Grammatical mood is one area that makes up for visual and arural deficits. Written expression of visual and aural sensations and a focal persona's attitude and therefore emotional mood toward them covers for those deficits.
Whose mood? Whose attitude? A narrator's? A viewpoint character's? I'm partial to a viewpoint character's, though a strongly held narrator attitude will do almost as well. Subconscious questions readers ask until they're satisfied they know the answer:
What is the persona's attitude toward the subject? Certain, even if from a logically flawed perspective? Then the grammatical mood might be the realis mood, or the indicative mood. "The indicative mood, or evidential mood, is used for factual statements and positive beliefs."  This post and many of my posts are written in an indicative mood. For example, Geraldine marched down Broadway
. Brooks no doubt, doesn't it? That latter sentence is in an irrealis mood, the interrogative mood. Note it's the verb marched that expresses both emotional mood and grammatical mood.
Is a persona's attitude uncertain? Irrealis moods express "necessity, possibility, requirement, wish or desire, fear, or part of counterfactual reasonings, etc."  And doubt. Other irrealis moods include subjunctive, conditional, optative, jussive, potential, and imperative. Though English verbs only express formally the indicative, subjunctive, and imperative moods, the other moods are often expressed through syntatical or periphrastical clause constructions rather than through inflected verb constructions. Or, actually, through aural inflections, tone of voice, and nonverbal expression inflections, such as gestures, facial expressions, and body language, in everyday conversations or expressed artfully in written word communications.
 Changingminds.org. Mehrabian's Communication Study. http://changingminds.org/explanations/b ... rabian.htm
 Wikipedia. Grammatical Mood. http://en.wikipedia.org/wiki/Grammatical_mood