Since the composition of shareholders changes daily, a specific moment is needed to fix the list for any corporate action. The right belongs to those listed as owners of the security at the end of that day, regardless of how long they held it or what happened afterward.
Separate dates are set for different purposes: to determine dividend recipients and to compile a list of general meeting participants. These dates do not have to coincide.
A key nuance is that settlement of transactions takes time. To be included in the list, a security must be purchased before the record date itself, accounting for the market settlement cycle. An investor who purchased a share on the record date itself will not be included in the registry.
Selling a security after the record date does not deprive one of payment rights: the dividend goes to those on the list, even if they no longer own the security when the funds are transferred. This is why the price drops after the cutoff—the new buyer acquires the security without the right to the declared payment.