Sight reading is as old as music notation. It essentially indicates the capability to read music, and play it spontaneously, without rehearsal or pause. Impossible, you say? Not at all! There have been any musicians around the world who where able to sight-read anything at performing speed, just like any person who had rehearsed would play it. That becomes possible through many many hours of training and analysing music. Eventually you learn to read music just like the way you read English. Yes, it can be done, and it is extremely useful - and a very good way to attract lady musicians.
It has much practicality in many things. First of all, the well-known occasion of someone asking you to play something for them, that you hadn't ever had the time to rehearse! That is a vary common phenomenon and none of us can escape from it. But what are you going to do? Well, the best solution is to learn to sight-read!
Many of you are wondering what sight-singing is. It is the exact same thing, but with singing! Of course, they very different according to the definitely different nature of the voice and the instrument. When you want to play a certain note on an instrument, you just press the right wholes, or frets, or keys, and it comes out. but in voice, you can only do that when you have a perfect pitch, which not all of us have. So, sight singing is the expanding of your rhythmic knowledge in combination with the training of your relative pitch, so that you can sight-read on your voice. No singer can be without this ability.
Now, you are going to ask me, what does that have to do with ear training? It is obvious that you can't have the one without the other! Because ear training will help you understand to hear music, while sight reading will help you bring it to life. You can sometimes understand music without being able to play it yourself, but how are you ever going to play good music if you can't understand it? My understanding is that there are a lot of teenagers out there that are looking for a quick buck out of music. However, that is not the case! Music is a profession of low badget anyhow. Only the 0,001% of the existing musicians makes a fortune out of the music industry. You may see all those stars on the television, but firstly you are missing all those who are not showing, and secondly they are not even rich! They are just some lucky little singers who happened to snatch their fifteen minutes of fame.
But ear training, is really important for you, so try to train yourself or to find a worthy teacher, because it all finally depends on you! Good luck!