קְדֹשִׁ֣ים תִּהְי֑וּ כִּ֣י קָד֔וֹשׁ אֲנִ֖י
The Ramban explains that this isn't a general command to not do sins, but rather a Mitzvah to withhold from an excess of what is permitted. For example, without this Pasuk, one could eat just until the point of gluttony, drink until the point of drunkness, etc. Someone who does this is called by the Ramban a נבל ברשות התורה - a degenerate within the permits of Halacha.
So, after Parashas Acharei, which lists many prohibitions, the Torah gives us a summary of these Halachos - to enjoy in moderation permitted things.
The Shelah explains that the reason the Torah doesn't say exactly what one may and may not eat, drink, etc., is because not everyone is alike, so we have a general command to withhold ourselves from too much of what's permitted, within our capabilities.
Similarly, the Admor Hazaken famously said 'What is forbidden is forbidden, and what is permitted - we don't necessarily need.
