We use an electric can opener? Why? Because in the last five years we have tried multiple manual can openers and every one of them has broken - even the high tech $25 dollar models. Some of the openers lasted less than two months. We've had an electric for a year now and it still works fine.
But if you want a useless and unnecessary invention how about bottled water? Not only is it incredibly expensive compared to tap water, but most of it comes from the same source as tap water anyway. Not only that but the plastic bottles are a major environmental problem.
Been hooked with videos on kitchen gadgets and it's unbelievable how many useless stuff people come up with. There's a banana slicer (hello, knife!!), and automatic spinning fork for your pasta, and those pizza scissors that actually make things harder.