But it is usually not necessary for approval of a compound to be able to describe how it works on a molecular or cellular level. What you need to show are three things: efficacy, safety and quality, so basically: the compound has the intended clinical benefit, has an acceptable safety profile and can be produced with a consistent manufacturing quality. Most compounds fail because of lack of efficacy (roughly half), and roughly a third because of lack of acceptable safety.
The vast majority of drug candidates don’t make it to the trial stage. Much of the research has to be defensible prior to the trial and what makes them defensible is having a mechanism for action. Of course once a drug is being used off label the starts to be some empirical data which can be used for trials, and it seems that we’ll get lucky with GLP1-As.