If you don't set a probability on the assignment then the probability that is set on the Pipeline step will be used. You set probability in a project when it departs from the probability set in the pipe stage.
Probability is used to calculate weighted value in the pipe step. It is also an important factor when calculating preliminary utilization for consultants in both Forecast and in the report Utilization report.
The probability is set when a new project is created or under Edit Project details.
The probability for the Pipeline Step can be editied in Administration -> Pipeline