The clip? It keeps the seal from falling off the coupler. If you've never had the seal come off all I can say is you must be a lot better at getting those tabs bent and secured than I am or somehow got a special one from the factory.
The u-joint? The stock couplers wear out, and have a certain amount of play in them anyway. The pins have to be pressed in, so rebuilding the coupler pretty much means pulling the entire steering column. And if there's any wear or damage at the pin there's additional play. And then of course there's just the fact that the seals always fall off, are a pain to reinstall, and the coupler wears out if the seal isn't in place. The U-joint takes care of all of that. New joint so less play in the system, the one I used has a DD end so no pin tolerances to worry about, and no complete failure if the pin breaks. Easy to replace in the future if any wear does develop, the U-joint can be replaced in the car with both the steering column and steering box in place. And in my case there should be enough movement in the telescoping section to switch back and forth between power and manual steering without adding an adaptor or changing steering shafts. Just buy a new U-joint and swap it in.