Phase I, II, and III trials are part of the “clinical development” world which exists to establish the drug as one that is functional and relatively safe for human use. Phase I trials are often done with animals or groups of around 20 people to determine what dose might end up being harmful or even lethal. Phase II trials are generally 50-80 people and occur in two stages. IIa trials focus on safe dosage as it relates to body response. IIb trials are larger and focus on drug efficacy. Finally, Phase III trials determine efficacy endpoints- if the drug shows weight loss, for example, then how much weight can one be expected to lose? These trials also seek to determine if the drug is more efficacious than a competitor. These trials cost hundreds of millions of dollars to complete, so pharmaceutical companies want to develop drugs that are novel and proven to be effective so they have the best chance of distributing them.
As a side note, my email here is AMYJ@novonordisk.com. Some random four characters from your first and last name end up comprising your email account, and out of all the other employees stuck with "DJOH" or "TYLA" or "PLEB", "AMYJ" is a pretty solid stroke of luck!