What Is the Practical Implication of Early Exercise for the Seller (Writer) of an American-Style Option?

The primary implication is the risk of unexpected assignment. The seller of an American option must be prepared to fulfill their obligation ▴ buying or selling the underlying asset ▴ at any time before expiration.

This forces the writer to manage their collateral and hedging strategies more actively, as the timing of the exercise is unpredictable and outside their control.