My understanding of modern cryptography is limited. Can someone explain if someone gets my horcrux files, would they be able to decrypt the original? If so, then what is the benefit of this tool?
If you give a plaintext file to a friend for safe keeping, the friend can see the file. You could encrypt the file, but encrypting a file requires a key or password, which you could forget or lose.
The idea here is: split the file into three parts. Give one part to the bank deposit box, one to a friend, and put one in a safe at home. Now, if you want to restore the original, nobody can restore without getting (for example) two of the three pieces.
This is nice because your friend and the bank can’t do anything with their copies alone, and won’t trust each other with their copy normally. But if you lose your piece, you can still ask the friend and the bank for theirs.
You could always encrypt the file before splitting it.