why is sound important in Tv drams

Sound is important because it highlights the main sequence of the scene, and heightens the reaction for the audience.
By adding sound it shows what type of scene it is. if its and action scene the sound would be fast paced and epic.
whereas if it is an emotional scene you would hear some type of slow sad background music to suggest or indicate that it is and emotional moment. Adding sound it shows that in different situations there could be different story lines.
In some cases sounds can tell and story without any dialogue. The way this is done is by matching the sound with the characters facial expression and movements. This shows that, sound can produce a story without and dialogue.

Also some sounds that are produced for the viewers maybe diegetic and non diegetic.
Diegetic: Diegetic sound is any sound presented as originated from source within the film's world
Non-Diegetic: Non-diegetic sound is represented as coming from the a source outside story space.

For example: if it is an nature show. most likely the sounds that are within the video would be Diegetic because the sounds are natural and the characters would be able to hear it. whereas if it was something science fiction like Doctor who some sounds would be non-diegetic. hearing the background only for the audience to see but not for the characters on set.

The reason why they do this is because they want to give the audience an hint to what would help them build suspense and create and illusion that something might happen to the characters.