I'm sure others have stated the most obvious and understandable reasons as to why subs are preferred over dubs.
Pertaining to me I prefer sub over dub.
However, if the anime was RAW and no one decided to sub it yet, then what to do?~
That is one of the reasons why I prefer subs. One, it's because you have subtitles helping you while enjoying the beautiful impact of their voices. Two, the subs allow people to learn Japanese by comparing the words spoken with the words written.
When you learn some of those words, you can watch some anime RAW. That is quite an achievement because you can then concentrate on watching the anime instead of focusing on both the subtitles and the animation.
That is the main reason why I like subs.
Dubs are pretty good in their own right depending on who's voicing them and whether it's abridged or not. They give a different flavor, but sometimes lose that 'feeling' of 'beauty' or 'impact' as the Japanese voices.
The English dub actors have emotion, but just not the 'right' one.
It could be because we tend to watch the sub first, then try out the dub. So we are used to the original.
In the end, I still respect the dubs. I just find subs more appealing especially when you realize that these sub groups are putting great quality work without much pay or no pay at all.