A cryptosystem is considered semantically secure if observing the ciphertext doesn’t reveal information about the plaintext (except, perhaps, the length; which all cryptosystems leak to some extent).
Semantically secure cryptosystems are essential for protecting sensitive data. Without this property, attackers could potentially obtain valuable information simply by observing encrypted data.
Therefore, it is important to carefully design and evaluate the security of any cryptosystem used to protect important information.
That is the exactly problem of the ECB mode.
https://words.filippo.io/the-ecb-penguin/
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_codebook_(ECB)