This was the biggest stuff-up done by Commodore. The Workbench 3.1 Install disk contains C:Installer but during the Workbench installation process that's one file which is never copied over! The handy Reboot command is also on the Install disk, but not copied over.
Anyone who installs Workbench should always manually copy those two files from the C/ drawer on the Workbench Install disk to the C/ drawer on their Amiga's boot partition. Reboot isn't used by much, it simply reboots your Amiga when you run it, but it's useful if your keyboard doesn't work well or you want a quick Reboot option to add to a ToolsDaemon menu or something. Installer of course is very essential for installing software!
Installer43 will probably work just fine, but if you ever run into an error in an install script try the original Installer instead.