This is mostly a matter of usage and what sounds more natural. And I'm writing from Portugal, so there's always the chance that someting sounds natural to me and not so natural to Brazilian ears. So here it goes. In spoken language you use the futuro mainly to express uncertainty or doubt.
(a) Parto para Londres amanhã, voltarei talvez no sábado.
If you you’re sure you’re coming back Saturday you’d sayvou voltar no sábado or volto no sábado. And you could say volto talvez no sábado too: the future sounds natural in (a), but it’s not compulsory. Similarly:
(b) Que dirá ele quando souber?
(c) Não sei se amanhã, depois de um longo dia de trabalho, estarei com disposição para o aturar.
You can use the futuro to express uncertainty even about the present:
(d) Ele já terá chegado? Estará em casa agora?
The questions above will sound rhetorical or implying that you are not confident the person you’re asking knows the answer. Otherwise you’d ask ele já chegou? Está em casa agora?
You use the futuro too to express a situation by a certain time in the future with the verb ser. Compare:
(e) Vou ser pai em janeiro.
(f) Em janeiro já serei pai.
In (e) you mean your child will be born in January. In (f) you mean that by January you’ll be a daddy already; your child may have been born earlier. It would be a natural structure in phrases like by the time you’re back, I’ll…
I reckon you rephrase (f) as em janeiro já vou ser pai, but it doesn't sound as natural to me. Structure (f) works with other verbs too:
(g) Quando voltares de Londres, já terei carta de condução; já poderei levar-te a passear.
(h) Daqui por um ano já estarei na casa nova.
But in (g) and (h) you could use ir plus infinitive instead.
I tried to sample my memory for usage of futures tenses in literature, but the problem is that stories, except for the rendering of direct speech, don’t use the future tense much. It’s more along the lines of once upon a time and what a good time it was there was a moocow coming down along the road… I suppose it will be more common in contracts and formal plans:
(i) As mercadorias serão entregues na sexta-feira e o fiscal inspecioná-las-á de imediato.
If you substitute vão ser for serão there is no change in meaning. It’s just that serão is more formal. Vai inspecioná-las is again more informal, and sounds like a forecast only, whereas inspecioná-las-á (= inspecionará as mercadorias) can sound like a prescription. (It may be the uncertainty conveyed by the futuro in informal language creeping in.)