"Please eat something."
Translation:Bitte essen Sie etwas.
That's just how the imperative of the polite form in German works. It is formed as "Infinitive + Sie", for example "Essen Sie", "Gehen Sie", "Schauen Sie" (for verbs without prefixes at least).
By contrast, for the informal forms, you don't use the pronoun in imperative sentences: "Bitte iss etwas" would also be correct.