I guess is someone messages you on Signal and you do not reply, or notice, it significantly damages Signal’s reputation with the person trying to reach you. They will give up using Signal and try to reach you in another way.

For the vast majority of people, not enabling notifications in a messaging app is a mistake in the sense of something they do want to do but failed to because they're bad at technology or just distracted, not in the sense of an ill-advised decision they made intentionally.