It's not necessary, but it sounds more natural when you add the redundant pronoun "se". Keep in mind that "se" isn't here the reflexive pronoun, but a special form of the indirect object pronoun "le" used whenever the direct object is indicated by "lo/la". In Spanish it's commonplace to insert the indirect object pronouns even when you specify what the indirect object is.

E.g. :A los estudiantes alemanes les dijimos que estudiaran más.     In this sentence, "les" refers to "Los estudiantes alemanes". It would be like saying in English : We told them to the German students to study harder, which is quite redundant, but it's the norm in Spanish

E.g. :A mí me gustan las fresas      This sentence is a more emphatic variant of "Me gustan las fresas"

I could add scores of examples, but I think what I said is enough to get the point. If you're interested in historical linguistics, then you'd like to know this unusual feature is one of the most frequently attributed to Basque influence in Medieval Romance, for in the latter language, either indirect or direct objects must be explicitly marked in the verb form used in a sentence, even when those objects are specified anywhere else.