If you back up your iOS devices in iTunes – which you should, together with iCloud backups – you can find some useful information about them. You may not need this information often, but if you do – if your iPhone is lost or stolen, for example – iTunes is a good place to look.
You can view iOS device backups in iTunes’ Preferences, on the Devices tab. You’ll see the names of your devices, and the dates of the backups. If you hover your cursor over a backup, you’ll see some information:
As you can see above, the information includes the phone number of the device (if it has one), it’s IMEI and MEID (again, if the device has these numbers; only devices with cellular access have them), and the serial number.
You may need the serial number if you’re getting support from Apple, and they may also ask you for the IMEI. So for a lost, stolen, or broken device, you’ll still be able to find this information. I recently called Apple about an iPhone 6s that I was selling, that I had erased, and was told that the IMEI was on the back of the device; this is no longer the case, and the Apple technician didn’t know that I could have found it this way in iTunes. (I didn’t either, at the time.) So it’s useful to be abel to get this information quickly.