One might wonder why there are two options:
1) Reading it from the scroll places the spell immediately into the spellbook.
2) No time required (other than the minuscule amount needed for the reading) and no ink or special ingredients.
3) Destroys the trace evidence (that is, opponents cannot steal the scroll; they'd have to take the whole book.) Arguably, this is not that great a trade-off, but if you're careful with your book (maybe in a portable hole, kangaroo pouch, or other ethereal travel device) then it because more beneficial.
4) Some wizards/casters have eldritch rules about their spells not existing on scrolls for others to find, and its generally a bad idea to piss off someone high enough level to make scrolls, as they tend to be quite powerful.
5) You might be a cleric or druid and not have a spellbook, so "some other" method is needed (see below.)
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Can you make a second (fully functional) scroll from an 'original'?
Sort of, but not directly. You have to copy it to your spellbook first, since you can only make scrolls of spells you know, and then, you can, if you're high enough level, create a scroll of the spell from your book. This is going to be a long process, though and expensive.
Where it gets weird is with clerics and druids, as they do not have spellbooks. In cases like that, the second (manual copy) is the only viable option, and they would, therefore, be able to simply duplicate it by hand... provided they were able to cast the spell in the first place. You could not, at 7th level, when you can first scribe anyway, scribe a scroll of a 9th level spell because you're incapable of casting the 9th level spell.
