Because extra security never hurt your data.
Have you ever gone to the mall or some other area that has a large parking lot, and when done shopping you couldn’t quite remember where you parked your vehicle? While it may be frustrating to wander around until you find your vehicle, at least you found it, and it was (hopefully) locked.
Think of your locked vehicle as being encrypted and your key is the decryption mechanism. Other people can see your vehicle, but only you can get into it because you have the key.
Nowadays most people understand that USB drives, or thumb drives, are ubiquitous due to ease of use and portability. Unfortunately they are also very prone to being lost or stolen. One of the first thoughts that probably comes to mind for someone that has lost a USB drive is “What data did I have on the USB drive?”.
With the parking lot example, your vehicle isn’t truly lost, it just isn’t where you remember it being. But more importantly it was locked. Generally speaking, data storage devices (USB drives, hard disk drives in desktops and laptops, etc.) do not encrypt data. This means that anyone can grab your USB drive or the hard disk drive out of your computer and see all of the data on it.
If your only response to that is “I’m safe because I have to enter a password to log in as a user on my computer” then your data is most likely not encrypted. It just means that you have a deterrence mechanism for someone that has physical access to your computer, which is extremely easy to bypass.
When your data is encrypted, even though other people may be able to get access to your USB drive or hard disk drive and see the encrypted data, they won’t be able to understand it because they don’t have the key (password) to decrypt the data. Just as someone may be able to walk up to your vehicle in the parking lot and try to open the door, they won’t be able to because they don’t have the key.
You lock the doors where you live and you lock the doors on your vehicle. Why would you not lock your data as well?