It's very important to warm up your body and voice before starting to sing. A gymnast would never consider doing back flips without warming up first. And it should be the same for singers. Always leave adequate time for gradual loosening and co-ordination of your muscles which contribute to vocal production.
Warming up will help wake the senses and get the blood flowing. If you don't warm up, your voice will sound harsh, your body will be tense and you could strain your laryngeal muscles.
