a) You have a slew of test inputs that all need to be tested through the same function, but you don't want to duplicate code.

b) You want something like "test with every combination of [x,y,z] for parameter a and [j,k,l] for parameter b". This is probably what the grandparent is referring to.

c)You have an even more complicated scheme, which you can define.

This is better than just having a for loop over a pre-generated list of parameters and calling a function with the same assertions, because pytest sees and handles them as separate but related test cases (e.g. when reporting errors, crash handling, fixtures, etc)