This is a nice overview of why you want to randomize. It also talks about block randomization and covariate adaptive randomization.
KP Suresh. An overview of randomization techniques: An unbiased assessment of outcome in clinical research. J Hum Reprod Sci. 2011 Jan-Apr; 4(1): 8–11. doi: 10.4103/0974-1208.82352. Available in html format.