It takes a bit for them to refresh, so by you grabbing it, that is one less item box for someone to grab behind you to get a bullet, star, red shells and all that goofy stuff. It will help you keep your lead. Also, if you are taking your normal fast line and grabbing the easiest ones, your fellow competitors may need to take wider corner or line, which will slow them down more to grab an item box.
You can also drop items behind you as you approach them to make room for new items, so you still keep the same number of items but not you have dropped some trash for them to work around or hit when trying to get their items. Often racers take similar lines, so you can throw them off by hoarding the easy boxes.
The only downside, is that Bananas & shells are good shields when dragged, if you drop a green shell and then end up with a fake dummy box, you lose your shield, so keep that in mind as you plan your item drop & grab strategy. Maybe it will help you keep the lead against your friends or gain you a few precious points on the WFC!