Holy water is not only a symbol of baptism. It is a sacramental, a thing specially blessed to do things. One of those things is to be “a means of health for those who believe, a medicine for body and soul for all who make use” of it, and “a purified water, empowered to drive far away all power of the Enemy”, “to root out and banish the Enemy himself, along with his fallen angels.” It is also meant to destroy contagion and anything else harmful to our peace and safety – though of course God may want to allow us to suffer certain things, so holy water doesn’t always “work” the same way in every situation.
Also, it reminds us of our Baptism, which of course is pretty spectacular, and thus it is powerful just to be reminded that we have become siblings and co-heirs with Jesus, and children of God. To be “reminded” in Christianity is generally to make something past somehow part of the present. It’s not pigs’ feet. So this is probably how it is that holy water remits venial sin. (Doesn’t let you off making reparations; but it’s good to clean off the venial sins.)
All in all, holy water is a big spiritual deal; and yet it’s just a sacramental, not a Sacrament. Don’t minimize it; maximize your appreciation for the Sacraments instead.
And I’m sure holy water is good for Protestants to use, if they respect it. You don’t have to fully understand a gift of God (who does fully understand?) to get good out of it.