Most, if not all, computer algorithms to produce a random outcome are in fact "pseudo-random". Typically the algorithm is "seeded" with a number on which more or less complicated calculations are done to produce an outcome that looks random. It is usually only necessary to provide a seed the first time the algorithm is called as subsequent calls will produce outcomes that look random. But, using the same seed number will always produce the same sequence of outcomes. It is therefore best to seed the algorithm with a number that is hard to predict. The number of milliseconds since the computer was turned on is usually a good seed.

For testing and debugging purposes it can be helpful to always seed the algorithm with the same known number, so that the outcomes are predictable. Maxis did this in producing The Sims 2. However, when releasing the final version of a program it is best to replace these known seeds with unpredictable ones. Sometimes unfortunately, Maxis left these fixed seeds in. (It makes things easier for support staff when users report problems). This is the case when determining the genetics of a Sim baby, resulting in the first-born syndrome.

mootilda could have explained this much better than I can.