Okay, this one is for the brass instruments and string instruments only. Long tones are a great way to build many things, such as tonal recognition and embrochure strength. The first one is the only part that shall apply to strings. If you play the same note for an extended period of time, it will be easier for you to recognize that note later, and it will be easier to find the exact spot on the fretboard. It is an especially good practice routine, especially for fretless strings. The same goes for brass instruments. In order to play an specific note, you have to make your embrochure (your lips) tighter or looser, and push the air. If you play long tones for an extended amount of time, you will be able to hear the note easier and remember exactly how to play that note. It will also help you with your range. I used to have loads of problems trying to play over the staff (tuba), but doing long tones strengthened my embrochure and made it easier to reach those high notes. A quick little hint: playing long tones makes your range increase, no matter if you play up or down. And the more you work on your low range, it will also help your high range as well. Happy Practicing!