Some questions have a single, straight-up answer. Others have several possible answers, and this question definitely falls into the second category.
One important reason for chanting concerns the breath and its close connection with our nervous system. Inhalation is connected to the sympathetic nervous system, which prepares the body to deal with some kind of threatening situation (witness that sharp intake of breath your mum takes the first time you drive her somewhere after passing your test…).
On the other hand, the exhalation triggers the parasympathetic nervous response in the body, and this encourages rest and repair on a cellular level, bringing the body back into its normal, relaxed state of functioning. When we chant, we extend the exhalation, which deepens this process, and this acts as an important antidote to the ‘fight or flight’ state of being constantly alert and on edge.
Another reason is that focusing on the sound and moderating the exhalation requires concentration, which in turn means that the mind’s tendency to bounce around from one thing to the next like a drunken monkey bitten by a scorpion (as described in the ancient texts) is kept firmly in check. In this sense, chanting prepares us for our yoga practice, and works like an energetic shower, washing off the chit-chat that often besets the mind and opening up the possibility for a steady flow of similar thoughts during the class.
As for why we chant in Sanskrit as opposed to any other language, here the reasoning becomes more subtle. There is a very important concept in the yoga tradition called prana. This corresponds with qi or ki from Chinese and Japanese systems, and translates roughly (and somewhat clumsily) into English as ‘vital energy’. Prana deserves its own blog post, but here let us simply say that it flows around the body through subtle channels called nadis.
These nadis are said to number 72,000, and those places where they meet are called chakras. We have countless chakras in the body but six are given particular importance – namely the six which sit more or less along the spine, from the base of the spine to the point between the eyebrows (the so-called crown chakra is not strictly speaking a chakra). The vibrations of Sanskrit, when pronounced correctly, are said to activate these chakras in a particular frequency pattern that has a highly purificatory effect.
So much for why we chant. Let us move on to what we chant, and specifically the mantra OM.
The word mantra is passing into English with the meaning ‘a statement or slogan repeated frequently’, but its original meaning in Sanskrit is much deeper. It is made up of two words: man, corresponding to mind, and tra, freedom or liberation. This could be translated therefore as ‘that which liberates the mind’. Of mantras, OM is held to be the primordial one, and indeed many other mantras begin with OM.
For those of us who have not grown up with an inbuilt cultural familiarity with the sacred sound OM, it is difficult to comprehend the scope of its significance. It is fair to say it is also hard to overestimate it. For example in the Mandukya Upanishad, it is written 'OM: this eternal word is all; what was, what is and what shall be', which is reminiscent of the Bible teaching ‘In the beginning was the Word … and the Word was God’.
Beginning with a low 'a' sound and ending with the light hum of closed lips 'mmm', the word OM is held to encompass not only all other words, but all language, indeed all possible thought (partly because these two sounds make up the first and last letters of the alphabet in Hindi and other Sanskrit-derived languages).
There are in fact four parts to the sound OM: A – U – M – silence. The first three represent the waking state, the dream state, and deep sleep, (or past, present, future; or again creation, preservation, destruction) and the silence after these three represents pure consciousness. That part of us which experiences all these other states but remains untouched by them – the silent witness.
Swami Sivananda writes that 'the sound of the bees, the sweet melody of the nightingale, ...the sound produced in the flowing Ganga, the sound that is caused when it rains - it is all 'Om' only'. He also tells us that when we chant Om, we should feel that we are 'absolute consciousness...infinite, unchanging existence'. (Bliss Divine, 7th ed. 2006, pp. 360-364).
As stated above, there are many possible answers to this question, and I’ve articulated just a few of them. I hope what you have read has been useful in some way and perhaps inspired further enquiry.