We use this custom function all the time, and we feel like it would be a great addition to FileMaker's official functions:
FileMaker's existing "value functions" require you to know the value number ahead of time, and then FileMaker returns the name of the value. But this function does the reverse: you give it the name of the value, and it returns the value number.
This function helps us quickly figure out the value number of a known value within a carriage-return-separated list of values. Especially useful if you're trying to take the user to a specific portal row, but the portal records may have sorted differently since the last time you were looking at that portal, so the portal row numbers have all changed since the last time you were there. This function can let you figure out where -- within a List of that portal records' primary keys -- your record exists. Then, you can simply jump to that portal row number. Much quicker than looping through portal rows.
Of course, what might be even better would be for FileMaker to enhance the "Go To Portal Row" script step, so that we can just quickly jump to any portal row based on a criteria that we specify. That would also be quicker than looping through portal rows.