ordinal data simulation

Dear all,
   I am doing an ordinal data simulation. I?have a question regarding the cut off values between categories. Suppose I have three categories, if I do regression, there should be two cut off values. I find some simulation code for the ordinal data. However, they usually only generate a uniform random number and compare the probability with the random number. Could any one be so kind to tell me the rational behind it. I don't know why we need a random number rather than  just  two fixed cut off values. 

Thanks very much!
Myckel