Do this. Do this today.
Even if you normally protect your user account with a decent password, that doesn’t truly protect your data if someone decides to swipe your device. For many computers, the drive can simply be removed and plugged into another system, or the computer can be booted from an external drive and the data can be copied to that drive. Android phones and tablets can be booted into recovery mode and many of the files on the user partition can be accessed with freely available debug tools. And even if you totally wipe your drive, disk recovery software may still be able to read old files.
Encrypting your local storage makes all of that much more difficult, if not impossible. Anyone trying to access your data will need a key to actually mount the drive or read anything off of it, and if you wipe the drive the leftover data that can be read by that file recovery software will still be encrypted even if the new data on the drive isn’t.
There are a few downsides. If you yourself lose the key or if your drive becomes corrupted, for example, it might be more difficult or impossible to recover data. It can slow down performance, especially for devices with processors that don’t provide hardware acceleration for encrypting and decrypting data. But, by and large, the benefits outweigh the drawbacks, and the slowdown for modern devices should be tolerable-to-unnoticeable.