Why ROT13 and Caesar Ciphers Are Educational, Not Secure
ROT13 shifts every letter 13 positions through the alphabet — and because the English alphabet has 26 letters, applying ROT13 twice returns the original text exactly, which is exactly why it's historically used to casually obscure spoilers or punchlines online rather than for any real security purpose. It's a special case of the Caesar cipher, which shifts by an arbitrary fixed amount instead of always 13.
Both are trivially breakable: a Caesar cipher has only 25 possible shift values, so brute-forcing every shift and reading which one produces sensible text takes seconds by hand, let alone by computer. The Vigenère cipher improves on this by using a repeating keyword to vary the shift amount per letter, resisting simple brute-force — but it's still breakable through frequency analysis once the keyword length is deduced (via techniques like the Kasiski examination), which is exactly why it was eventually superseded by real cryptographic algorithms once computers made manual cryptanalysis at scale practical.
These ciphers matter today purely as an accessible, hand-traceable introduction to substitution cipher concepts — not as anything resembling real security for actual sensitive data.