A job. A thousand hours. Experience.

Perhaps you simply think that the pinnacle of life is to be a 121 captain.

Has the 135 SIC been paid? Then (s)he got paid, perhaps benefits, perhaps exposure. Perhaps greater total experience or time that leads to a particular job.

At this moment in time, it may be that a little self-entitled curtain climber can fall into a 121 cockpit with nothing but hope and bare ATP qualifications, but this has not always been the case, and it will not always be the case. Perhaps you lack the experience, or perhaps you've been in the industry for too short a time to know this.

1,000 hours of time, PIC or SIC, may be recognized by insurance, or an employer. It may be the road to the PIC job in the 135 operation. It may be a lot of things. It's still a thousand hours.