Hitting the Attack button with good timing just before an item arrives can catch it, turning a potential incoming threat into a tool for you. In modes with all the items enabled and lots of opponents, there are plenty of opportunities to try out catching tossed items. (Also remember to use any unique moves your fighter might have to absorb or reflect items and shots.) Just keep in mind that catching also whiffs an attack while snatching the incoming item. An adversary who’s thinking ahead and knows you’ll go for a catch might chuck something your way, then rush in, waiting to whiff-punish when you swing for the catch.
Even outside item-heavy modes, certain fighters can produce their own items at will with special moves. So catching is useful beyond chaotic free-for-alls, even if items are disabled.
Some fighters get a lot of mileage out of tossing and catching their own items, like Peach and Daisy, the various Links, and Diddy Kong. Catching your own tossed item is much easier, because your fighter can overlap with the item before your catch, increasing consistency and ease. If you wait that long when, say, Snake tosses a Hand Grenade at you, it’s already bounced off and detonated. You can catch your own items in midair without whiffing an air attack by pressing the Grab command, Attack+Shield, just as the item is about to hit your fighter. If possible, use a button assigned to the Grab command, which is far more consistent—there’s almost no leniency for this trick if you hit the Attack and Shield buttons even slightly apart. With the Grab button, it’s down to proximity with the item.