The Saibaimen were seeds brought by Nappa when he and Vegeta came to conquer the Earth. When Nappa's seeds were planted, they grew into these Saibaimen. Their sole purpose was to fight, and in fact Vegeta trained on his home planet by fighting these brainless creatures.
With a power level of 1200 each, the Saibaimen were worthy opponents for the Z fighters. Yamcha was killed by a Saibaimen, but most of the Saibaimen were killed by Krillin. Vegeta grew bored and killed the remaining Saibaimen.