Hearing a pianist play your song or aria before you take it to an audition is important. If you don’t read music, this is even more important. You may erroneously assume that your song is the exact same version that you heard on the radio, so it may come as quite a shock when you hear your song for the first time at an audition and have no earthly idea what those sounds are.
Remember that publishing companies usually publish songs in keys that are easy to play. If the singer on the radio sings the song in a really hard key with many accidentals (sharp, flat, or natural signs placed before individual notes to indicate that they’re a half step higher or lower), the publishing company may change the key to make it more accessible to beginning pianists.
Singers who also play guitar may really enjoy accompanying themselves for an audition. Adrenaline may cause your hands to shake, so practice in front of an audience before the audition to work out the nerves.
When you audition or practice with a guitar, you want to maintain your alignment, coordinate your breath, and look at your audience. You can look at your guitar, but look up frequently so that the audience can see your eyes and connect with you.
By having a pianist read your music for you before an audition, you get an opportunity to check the key to make sure that it’s exactly in the range where you want to sing.