This subject is really complex... I will try to summarize. I hope this information can be useful to you:
"LHE" is an oblique pronoun used to replace the complement of a verb that requires the preposition "a" or "para". "LHE" always refers to people (not used for things) and can be used instead of "ele"(he), "ela" (she) and "você" (you). Examples:
- "Por que você está surpreso? Disse-lhe que viria."
[Why are you surprised? I told you I would come]
- "Encontrei o João. Então, entreguei-lhe o convite."
[I met John. So I handed him the invitation]
Notice that in the phrase 1 the "disse-lhe" is used instead "disse a você". And in the phrase 2 "entreguei-lhe" is used instead "entreguei a ele".
So remember, you use "lhe" when the verb is "transitivo indireto" (the oblique pronouns to the verbs that don't use prepositions are others, but this is another history) and it applies only to the third person ("ele", "ela") and for the special case of "você" (that is polemically treated as third person).
You can notice that "lhe" is very important and it keeps your phrases simpler and less redundant. The alternative would be to repeat the complement, but use the "lhe" will sound more natural.
NOTE:
Maybe is a question of habit. For example:
1) If you talk to people that use "tu" instead "você" they will say "lhe" les frequently (as they will use it only for "ele" and "ela")
2) Some people in Brazil, even when they prefer to use "você", they use the "tu" oblique form "te" (as "tu" and "você" are considered synonyms in Brazil). But this "alternative" is a bad idea, they can't run from use "lhe" for "ele" and "ela" and they have the risk of mix "você" and "te" in the same phrase and this is wrong.