Being new to the sailing of catboats, I have not yet figured out the role of the topping lift other than the lifting up of the far end of the boom high enough such that no one has a head injury.  I presume that there are more intricacies than that.  Any details would be much appreciated!

I use the topping lift to arise the aft end of the boom so I can remove the boom crutch prior to hoisting the main, then slack it off so it doesn't affect the shape of the sail. On a small daysailer equipped with lazy jacks you might be able to dispense with it completely.

Catboat and How to sail her, CBA bookstore.
Reefing, take tension off leach, easier to tighten outhaul .
Downwind in rolling waves, raise boom to keep it from tripping in waves.
Scandalizing, to shorten sail in a hurry by easing peak, top up to keep boom up.