It's symbolic. It is a formal and official recognition of the injustice, and a gesture of amelioration - at least for the public record. Though too I presume his descendants will consider it a a family victory as well.

The reason no pardon has been granted is that a pardon is just that it basically acknowledges guilt. Johnson's conviction should have been overturned.

Okay, it should never have happened, but a pardon is not appropriate.

Not helpful for him at all. He's dead. Perhaps it may make his family feel better and maybe get Trump some "African American votes.