That is only one, the most basic purpose and the only that self-signed certificates can do. LE and other certificate authorities* at least prove that your data

That is only one, the most basic purpose and the only that self-signed certificates can do. LE and other certificate authorities* at least prove that your data wasn't tampered with in transit, or wasn't decrypted, read and reencrypted without any way for you to find out. * as long as you trust those authorities