Why the form is chosen
Peptides in solution degrade. Hydrolysis, oxidation and aggregation all proceed faster in water, and the rate climbs with temperature. Removing the water slows all of it dramatically, which turns a product that would need cold chain into one that tolerates ordinary handling.
What the cake tells you
The dry material in the vial is called the cake. A well‑formed cake is uniform and holds its shape. A collapsed, shrunken, or melted‑looking cake can indicate the vial was exposed to temperature or moisture, though appearance alone is not proof of anything and a small or barely visible cake is normal at low fill weights.
After reconstitution
Once liquid is added the stability advantage is gone and the clock starts. Storage conditions and useful life after reconstitution are properties of the specific compound, not a single universal rule.