in Editing
Reading your manuscript aloud, and what it catches
Your ear notices what your eye forgives. Reading a manuscript aloud is slow and slightly embarrassing, and it finds problems nothing else does.
Silent reading is fast and forgiving. The eye skims, fills in missing words and smooths over clumsy rhythm without reporting it. Reading aloud removes that help. Every word has to be said, in order, at the speed of speech.
This is why so many writers and editors use it as a late check. It is slow, and it can feel faintly ridiculous alone in a room, but it catches a particular class of problem very reliably.
What the ear catches
The most obvious catches are mechanical. Repeated words, especially ones a few lines apart, are much easier to hear than to see. So are missing words, doubled words and sentences where an edit has left two versions of a phrase side by side.
Rhythm problems come next. A run of sentences that all have the same length and shape will start to drone when spoken. A sentence that goes on too long will leave you short of breath. A paragraph that ends on a weak word will sound as if it trails off.
Then there is dialogue. Dialogue that reads well silently can sound stiff or overexplained when spoken. People rarely address each other by name in every line, rarely speak in complete, grammatical paragraphs, and rarely explain things both parties already know. Hearing it makes this obvious.
Finally, reading aloud exposes unclear sentences. If you stumble, reach the end and realise you parsed it wrongly, or have to go back to find the subject of a verb, a reader will do the same.
What it does not catch
Reading aloud is a sentence-level tool. It will not tell you whether a chapter is in the wrong place, whether a character's motivation holds up, or whether the argument of a non-fiction book is sound. Those need a different kind of reading, usually with notes and a view of the whole.
It is also a poor check on spelling and on the consistency of names, dates and style choices. You will say a misspelled word correctly without noticing. A proofread, or a style sheet, is the right tool there.
Doing it properly
The method matters. Read at a normal speaking pace rather than a mumble, because mumbling lets you skip the same things silent reading does. Read from a printout or a different screen layout if you can, since a change in format makes familiar text look new.
Keep a pencil in hand and mark anything that makes you hesitate, without stopping to fix it. The point is to gather a list of places that need attention. Stopping to rewrite breaks the flow you are trying to hear.
Some writers use text-to-speech software instead of their own voice. It has one advantage: it reads exactly what is on the page, with no unconscious corrections. It has one drawback: its rhythm is not a human one, so it is less useful for judging how prose sounds. Many people find it worth trying both.
When to do it
Reading aloud is most useful once the structure is settled. There is little point polishing the rhythm of a chapter you may cut. Late in revision, once the big decisions are made, it is one of the best ways to find the small problems that remain.
It takes hours for a full-length book. Most writers who try it think the hours were well spent.