Yes, Time Machine does much that iCloud does not. Time Machine backs up everything on your Mac - all configurations, preferences, apps, user accounts... You can get your entire computer back with a minimum of fuss if you need to erase or replace the HDD or move to a new Mac. Sure, it can be time-consuming - a Time Machine restore may take all night, but you can walk away instead of spend hours at the computer trying to get things back the way they were.
iCloud stores specific user data, if you choose - the Desktop and/or Documents folders, iCloud Photo Library, iTunes library (if you use Apple Music or iTunes Match), plus Contacts, Calendars, Reminders, Notes, and the like. That may be enough for your needs, but you may have a lot of additional work to do if you need to recover from a disaster - reinstalling apps, reconfiguring preferences, getting your Dock back to the way you like it... If you have a POP mail account, you've lost your message archive. If you don't keep all your photos in iCloud Photo Library or you placed files in your Public folder, if you failed to move documents and data from Downloads into Documents or Desktop... you're SOL. If your computer has multiple login accounts, are you sure all the other users have similarly enabled all those iCloud features?
As Joni Mitchell wrote and sang, "Don't it always seem to go that you don't know what you've got 'til it's gone!"
