Specifics: I can find no evidence whatever that ‘hails from’ is ‘necessarily formal’ in any particular form of words, in any instance of English anywhere. That does not mean that it does not figure in some process somewhere (maybe English-speakers getting mortgages in Bangalore have to use this term in declaring where they are from), but it does not indicate a conventional change to a formal register.
‘Hails from’ is commonly applied to people and their backgrounds, but it can legitimately refer to the relevant or influential origin of anything.
In my experience, ‘hails from’ signals that someone’s identity and/or character is in some sense derived from that place. It is much stronger than ‘comes from’, which could mean no more than ‘is currently camping in’.
To me (long-term student of many Englishes, and UK inhabitant) the expression ‘hails from’ in that sense feels comfortably traditional, rather than archaic. With that in mind I wondered whether I might find it in Tolkien, so I went hunting online.
This delivered no direct quotes from the Lord of the Rings books (I bet it’s there!), but unexpectedly it revealed a calmly consistent and current usage across many websites indicating the general sense of influential or notable personal history that I had in mind.
The emerging central point seems to be that coming from somewhere can be a simple matter of convenience or historical accident. Hailing from somewhere contributes (however unspecifically) to defining you... otherwise you wouldn’t phrase it like that. We don’t know why Mirkwood is relevant to Legolas being ‘valiant’ and ‘a great archer’, but we have the impression that it has been somehow conducive to those qualities. We don’t know exactly why Acevedo doesn’t simply say he lives in LA and has done for ages, but for some reason he is also careful to let us know that he hails from Arizona.
In the absence of positive evidence of formality...
The designation ‘formal’ is probably misleading here, implying that ‘hails from’ introduces some kind of ceremonial significance to conversation in British English. It has no such general cultural weight, that I am aware of. In this kind of case, ‘formal’ seems to mean that everyone understands it but would normally say something more vaguely casual.
Of course there may well be established forms of words where it is used as a matter of form and habit (perhaps the Best Man conventionally ‘hails from’ somewhere in certain wedding formats), but ‘hails from’ does not automatically signal any specifically formal approach or tone (with a proviso for exaggeration or irony, below).
On the other hand, ‘hails from’ does have a generally weightier significance than ‘comes from’.
Someone introducing me could reasonably say any of the following, as a general way of helping people to get an initial handle on me:
‘He comes from London’ (because that was where I was born)
‘He comes from Bristol’ (because that was where I went to school and
‘He comes from St Andrews’ (because that was where I first studied,
got married, and lived for a while)
‘He comes from Milton Keynes’ (because that is where I currently
The third of those is a bit of a stretch, but it has happened, and any of them could be adopted to give a given group an appropriately helpful perspective.
Hails from, however, conveys a much stronger and more singular sense of belonging and connection. If you tell people that I hail from somewhere, you are telling them that I emphatically bring with me something of that place rather than any other, as part of my character.
If you say of someone, ‘He comes from Yorkshire,’ you basically just convey that that gives us something to talk about (birth, study, whatever). It might be gigantically significant (‘He comes from Mars’ works fine), but it need not be at all.
If you say, ‘He hails from Yorkshire,’ you are saying (whether or not you intend to) that that is an active part of this man’s identity. That heritage is going to form a noticeable part of how he interacts with us.
The proviso... Humorous use of ‘hails from’ is possible, making it seem more generally formal than it is, comparable to someone shouting ‘Greetings!’ when you enter a pub. (That, too is ‘formal’ in the sense that it is transparently understood but not commonly used. It sounds a bit like something for special occasions.) In such an atmosphere, saying that someone hails from Wootton Bassett is tantamount to saying that this by-the-way formality is frankly unimportant, and now that we have it out of the way we should forget it and get on with drinking.
Also... ‘Hails from’ need not refer to individual human background, although that is the sense in which we usually encounter it. For example, Thomas Hardy's poem ‘The Fiddler’ includes the line ‘Music hails from the devil’ (in describing the fiddler's function of driving a devout wedding ceremony to debauchery).