While reading, I tend to subvocalize what I have read. I feel this as disturbing, but as many people do this, there must be a reason why the brain does this. So, what is subvocalization good for?
It should be pointed out that subvocalisation while reading is not necessarily of the same cause or purpose as subvocalisation while doing other activities. Many people are able (or accustomed) to comprehending written language only by converting it to spoken language mentally. This allows the interpretation process to share the same brain regions, as opposed to using one modality for reading and another for listening. Naturally some people are more prone to logical (read: serialised) thinking while others are more prone to visual (read: parallel) thinking. Those individuals prone to logical thinking are more likely to subvocalise while reading since this allows the reading process to correspond well with one's usual way of thinking. The reverse should be true for those accustomed to visual thinking.
The act of subvocalising while doing non-reading activities is probably similar in that it may allow the thinking to be of one's usual modality (logical). Taking it further, however, subvocalisation is a way of putting thoughts into words. It is common knowledge that when you put an understanding into your own words you are more likely to develop a lasting memory. This is presumably because explaining something in words requires that the ideas be organised in a coherent fashion, as opposed to possessing merely a superficial glimpse of awareness. To think about it another way, putting thoughts into words creates durable mental objects that can then be stored away into memory.