Yes, it's worth it, by far.
Computers get stolen. If the hard disk is encrypted, the thief ends up only with hardware, and hardware is cheap to replace. If the disks aren't encrypted, the thief has the hardware and the data. Depending on what kind of data, the company can lose intellectual property, industry secrets, HR data, and that can translate into huge fines (or dead people, if the data is the real names of CIA operatives).
USB devices are easy to forget, or to misplace. If they are encrypted, and the key is stored on the computer, there's nothing on the drive that can be read.
Full disk encryption indeed keeps the key on the system, but the key is password protected, and the password is not on the system. Bruteforcing the password is possible, but depending on the length of it, can take way more time than the data is interesting (think taking 1000 years to crack it).
High security systems keep the keys on a TPM or a smartcard. The first has pretty secure settings, and capturing data from inside a TPM is not remotely easy to achieve. And a smartcard usually is not stored with the computer, and usually is protected by a PIN and auto erases itself in case of a bruteforce attempt.
There are LUKS settings (for Linux computers) that don't even keep the encrypted header on the disk, but it resides on a USB drive. In the case of theft, there is nothing to be bruteforced, because the header is on another hardware, probably with the user.