Most computer now days that you would get at the store come with just about everything you need to operate your computer for day to day use. Some of those key I/O devices are a keyboard, so you can enter things such as text lettering into browser bars for web searching. Another is a mouse, which is used to select files or applications that you want run. Another key one is a monitor, without that you couldn’t use either one of the previous stated I/O devices because you couldn’t see what you were doing. The last one might be speakers, not all come with them, but without them you wouldn’t be able to hear any sound playing or hear any kind of voice in videos.
Some I/O devices that are not necessary, or that are not considered day to day uses might be. A printer, I know I have a printer and I do not need it all the time, it could be days if not weeks till I use it. Another may be a fax machine. I’m sure most homes don’t have them, unless you’re running some kind of business, but using it for day to day use is unlikely. Another might be a web cam, most people will use their cell phone to do a video call, or again if you were a big business I could see where you would need one. But for the average American I wouldn’t see the need for one.
A modem is considered an I/O device because it would be your main source for data transfer. It’s how people communicate worldwide via e-mail, picture mail, paying bills the list goes on. Without a modem you couldn’t connect to the internet. A printer could be classified as an input device if it was an all in one printer, meaning a fax/scan/print machine. It has a lot of options with sending files to your computer, to printing out a paper, or by connecting it to a modem, you could fax important documents to someone five states away.