Every chain starts with a block that has no predecessor. It is written into the client software, and it fixes the initial state: which addresses hold what, and what the rules are from block one.
Reading the genesis allocation is basic diligence on any new chain. It shows whether supply started fairly distributed or concentrated with founders and early investors, which tells you who the future sellers are.
Example: a chain whose genesis assigns 40% of supply to team and investor addresses subject to a vesting-cliff has a structural overhang. Compare that with the circulating-supply and the token-unlock calendar before treating a low float price as meaningful.
Related: block-height