[AngelicaDeAlquezar]

The use of pronouns can change from region to region, but in Mexico it's necessary in most cases.

If you're using Juan's name and just want to order he's been hit with a hammer: Golpea/golpee a Juan con un martillo.
In this case, Juan is a direct object, so there's no place for an IO pronoun: golpéelo/golpéalo.
If you specify the part of his body to be hit, you're doing something to him, so the pronominal is needed: "Golpéale/golpéele la cabeza (a Juan) con un martillo."

As for the raining example, if you already know whom you're talking about: "Dígale/dile que llueve / que está lloviendo."
If there may be confusion with the subject, then "a él / a Juan" is needed and many people would find that the pronoun isn't: "Diga/dí a Juan que llueve", but in Mexico it is needed: "Dígale/dile a él que llueve / que está lloviendo."