Much like in real life, friends in Sims 4 usually serve no purpose other than to fill social needs. It's a refreshing departure from previous iterations of the game where they were needed for career advancement and what not. Granted, there are still aspirations and careers in TS4 where they are needed as well, but those are few and far between.
Worth mentioning though that being friends with your co-workers helps career advancement in all career fields. Helps being the key word, they aren't a necessity, just make promotions faster.