Subtitles are needed for promotional video simply to make them understood by as many people as possible, that means hearing impared people, as well as those that does not understand the original language of the movie and many of those that their sound system isn't good enough. Here is how you do it and links to get the tools.
A good video require subtitles for several reasons: First, not everyone speaks your language, second, not everyone can hear properly due to his/hers sound system, and third many people are simply hard of hearing or deaf. Therefore if you would like your video to be understood as well you better add subtitles. In order to do this you need a software.