Linux 101 stuff. Questions are encouraged, noobs are welcome!

Anyone with physical access to the computer that's unencrypted can see all of the data. If that's all right with your threat model then thats ok.

Having full disc encryption means the data is not available unless somebody has the password, usually guarded by a secure element in the computer itself that limits the number of unlock attempts to something reasonable. So if the device is stolen or copied, the data can't be used..

To take the opposite position for a second, even if your server has full disc encryption, if it's online and attached to a network and unencrypted, it's still unencrypted. Well the machine is on and doing useful work the full disc encryption only applies to data at rest, anything inside of the operating system sees things unencrypted (I'm talking about the general case here, obviously there's other things you could do to ensure data is encrypted while the computers on but that's not what full disc encryption usually means)

If the hard drive isn't encrypted you can always physically remove it and connect it to another system to access your data.

An encrypted hard drive means that someone cannot physically steal your hard drive and read its contents.

Encryption-at-rest is generally moot against RCE exploits, because your OS will happily decode files that your programs have permission to read.

That said, on modern systems, encryption is cheap. So set it up if you can.

As others have said drive LUKS is primarily meant to protect against someone getting your data if they physically steal your device/drive. This is less of a risk for a home server then say a laptop or phone.