If you consume a balanced diet and receive 100% of each of the nutrients your body needs on a daily basis, you probably wouldn't need or want to drink Soylent. But if you want to maintain precise control of your dietary intake, Soylent is a great way to do it.
Soylent has been reported by many, including Rob Rhinehart (the inventor), to have substantial health benefits. However, these benefits are only likely to be found for those currently on a 'bad' diet.