Musical instruments have but one use; making music. A guitar, ukulele, piano or violin is used to make music, and that is its sole or core purpose. It might be used as a trade item, but ultimately, its use is in making tunes.
Are you asking about orchestration or instrument classification? If so, you worded it pretty poorly. If you worded the question exactly as you meant to, then...yeah...pretty "duh"-ish. However, Google the instrument(s) you are curious about for an idea of when a composer would choose to use them, and for some musical excerpts in either notation or mp3 format. Or just read Berlioz's "Treatise on Orchestration."