If bottle was used to feed baby formula, does it still need to be sterilised? If yes, why?

It’s best to continue sterilising your baby’s bottles until baby is atleast a year old because they are vulnerable to the germs. These germs can develop very easily in leftover milk especially those that might be collected in bottles unknowingly. Sterilising is the only way to make sure baby's bottles are free from harmful bacteria. If baby is old enough, sippy cups or beakers might be better for your baby's teeth than bottles. They also don't need to be sterilised, because they're easier to clean.