Spanish language Q&A Forum

The following two sentences use the indirect pronoun 'le' before the verb. Why is 'le' necessary in these cases? Is it considered incorrect if it's removed? Examples with and without 'le' are below. 

1. Tú les das vino a mis abuelos.

2. Ella le muestra un sombrero nuevo al hombre. 

I've heard native speakers omitting 'le' in similar sentence constructions, but I'm unsure if it's correct to do so or not. 

1. Tú das vino a mis abuelos.

2. Ella muestra un sombrero nuevo al hombre. 

 It says ' Do you prefer to wait sitting or standing' and I had to fill in the gap for prefer/preferir.

 ' Usted prefiere esperar de pie o sentado?' 

I don't understand how prefiere is the answer and not prefieres

“María’s family are happy” is given as the translation to “La familia de María está contenta”. 

This didn’t sound right to me so I googled and found this- https://style.mla.org/verbs-with-collective-nouns/

The reference would suggest that the translation should be ”María’s family is happy” as the members of the family are in agreement. 

Any comments would be helpful. Thank you.