Reason 1: You can save them time, energy and reputation.
Reason 2: You can do something better than they can.
A job hiring for reason #1 alone is mostly clerical, mindless, and redundant. You are a cog in the machine and mostly expendable. Avoid this kind of work if you can. I know you have more value to the world than that.
A job hiring for reason #2 alone is more privileged, thoughtful, and independent. You bring a lot to the table as an accessory and may be missed when gone. This kind of work earns more respect and is far less expendable.
A job hiring for both reasons is more appreciated, integrated, and reputable. You are important to the operation and can damage the greater whole if you leave. Your stress and accountability to others may be greater, but your value to the company can leverage higher reward.
Know why you’ve been hired. It’s important to contextualize your value to the company.