Charging is a lossy process. Charging a battery, then using it to charge another battery isn't as efficient.
Plus, depending on how/where it's used, having to wait for it to recharge while connected to a power bank might be a non-starter. You also don't necessarily want to recharge while transporting in a bag either because of heat concerns.
Plus, depending on how/where it's used, having to wait for it to recharge while connected to a power bank might be a non-starter. You also don't necessarily want to recharge while transporting in a bag either because of heat concerns.