To make you read faster - Reading out loud slows down your reading speed. If you usually read out loud you will start reading out the words in your head even when you are reading silently and so reduce your reading speed all the time to the speed when you are reading out loud.
To improve understanding - Because half of your brain is concentrating on pronunciation when you read out loud, you comprehension is much less than when you are reading silently.The same is true when listening to other people reading out loud.
To really concentrate on pronunciation - In the same way as point 2 above, if half your brain is thinking about the meaning of a text then you won't be able to put your full effort into your pronunciation. Also, it is impossible to concentrate fully for the length of an entire text, so it is much better to concentrate on one or two sentences and practice them over and over in different ways.
To help you ignore words you don't need - When you are reading out loud you have to think about and pronounce every word in the text, but there are many words you don't need to be able to pronounce and/ or understand, such as people's names and place names. When you are reading silently, you can just skip past anything that you think is too difficult or not important, and then go back to it later if you need to.
To allow you to reread - If you are reading out loud, once you have read a word or sentence with the right pronunciation it is normal to move onto the next part. If it was an important and difficult to understand part, though, it can be worthwhile to read it once or twice more. This is much easier and quicker when you are reading silently.
To help you read whole words at once - When you are reading out loud it is normal to read each word from the first letter to the last in the way it is pronounced, but it is possible to read faster by looking at a whole word or even groups of words at the same time and then moving onto the next section.
To move quickly to the information you need - In the same way as point 6 above, reading out loud means reading each sentence in order. With most comprehension questions in exams and textbooks, it is much quicker and easier to read the question and then quickly skim and scan until you reach the part of the text where that information is. You can then read that part slowly and carefully, and as many times as you like.
To involve all the students - It is very difficult to concentrate when another person is reading out loud, and even if you are listening carefully it is not good listening comprehension practice as, unlike real life, you have the text in front of you to read too.
To give a good model - Other students are not usually a good model of pronunciation and speaking at natural speed, so listening to them read out loud is not likely to improve your pronunciation and listening skills.
To help with exam practice - You cannot read out loud in an English language exam like FCE or TOEFL, so you should stop all reading out loud if you want to take one of these exams.
To stop you moving your lips - Some people who have done most of their reading out loud move their lips as if they are speaking even when reading silently. This slows down your reading speed and can be embarrassing if people see you!
To allow choral drilling - It is impossible for a whole class to read a long text out together, so if the teacher wants the whole class to loudly practice their pronunciation it will have to be with something shorter such as a dialogue.
To help your confidence - Reading out loud well is a very difficult task that even some native speakers can not do in a way that people would want to listen to. Reading silently and doing pronunciation with shorter passages makes both skills easier and more enjoyable.
To help your listening comprehension - Texts and people you will need to listen to will be much faster than you reading a text out loud, so reading silently and quickly is actually better practice for real life listening comprehension than reading out loud is.
To give realistic pronunciation practice - Written language and spoken language are very different, so a sentence from a magazine is not good practise for the linking, weak forms etc that are found in natural speech.
To give effective pronunciation practice - Many students who can pronounce well when reading out loud find that their pronunciation is much worse when speaking freely, which suggests that reading out loud is not a good way of improving pronunciation. Activities like minimal pairs, shadow reading etc are much quicker ways of improving your pronunciation during normal communication.
To concentrate pronunciation on one thing at a time - A reading passage might have examples of every single sound in English and every example of how sounds change in fast speech. It is obviously not possible to learn all these in one lesson, so for pronunciation practice it is much better to use carefully selected words and sentences with lots of examples of the pronunciation point you are practicing that day.
To give a variety of reading tasks - Many fun and useful reading tasks like jigsaw readings (when different people read different texts and then compare their answers) and reading races are not possible if people read out loud.
To give a variety of pronunciation tasks - With the time your teacher saves by not using reading out loud, it is possible to do lots of fun and useful pronunciation tasks such as phonemic symbols, crosswords and identifying sentences when they are hummed without words.