Fixed - when tasks have a fixed cost. You outsource the task, and the outsourcers calculate the price before the work begins. The advantage is that the cost is known right away, it cannot change. As a rule, if the outsourcer does not calculate it correctly, or take something into account, this is his problem.
Minus of the fixed-approach - the cost includes risks, so it is usually 10-30% higher than the hourly rate. Also before the start of work, time is spent on evaluation: the task is divided into subtasks, the responsible experts determine how long it will take, what kind of expertise is needed, then managers estimate the cost. If you send tasks to outsourcers in a stack of 20-30, the time losses can be large.
There is also no way to choose the level of specialist, his or her qualifications are determined by the outsourcer. This is not a drawback, just a little nuance, because in this case, you pay for the result, no matter who will do the task.