Importance of Selective Attention

Selective attention is the ability to ignore all unimportant information and focus more on the information that you are trying to receive. This especially helps a lot in a situation where there is a lot of action going on. Say someone is trying to talk to you. If you do not have selective attention, you would have a hard time listening to what he/she is saying because all the sounds around you would be pounding at you from all directions. This applies to my life because I have been a musician since the 3rd grade. Being able to listen to the key parts in the music has helped me become a better musician. For example, there are parts in music where the trombones are important. Being able to single out the trombones enables me to figure out where I have to be quieter so the trombones are heard by the audience. I do have a question about this topic though. It says in the textbook that people randomly hear information from a conversation that he/she is not even involved in. Why does our brain do that?