Signal is a system designed always to be encrypted with no plaintext opt-out, with secrets that change automatically over the life of a conversation (which could last years) so that a single point-in-time loss of secrecy has minimized damage, with no exposed message metadata so you can't accidentally betray your conversation with a dumb subject line, and with deniability so that someone wiretapping you can't cryptographically prove you to have said anything.
PGP and email is essentially the opposite.