I use them to organize data as well. But you can do that (depending on the PLC) with structures or UDTs also.
Sometimes repetitive logic can be broken down to a few lines of code when you create the functionality once and feed it with data that is organized in an array. You could create a loop that increments an index and execute the same functionality with different data, adressed indirectly with that index.
There are many tasks that can be simplified using arrays, the first application that we were given in a PLC class was to create a program that would write a measurement from an analog input into an array acting as a FIFO buffer. You can expand that program more, you could search for the min/max value, calculate the average over x samples and so on.
