Target audience

You work in tech in the United States and you're switching jobs.

You can negotiate with your current company too, sometimes with more leverage, but that's out of scope of this piece.

Why negotiate?

The obvious reason: More stuff is good! Compensation is important to your standard of living today. Separately (even if you can afford to live in San Francisco in the same apartment on both $80k and $180k), cash and especially equity can help dramatically expand your set of lifestyle options if you join a startup and it grows a lot.
For some (especially senior and/or non-technical) roles, the role itself (title, responsibilities, future team size) may be negotiable, which could affect whether the position appeals to you.
Better understand how much the company values you, and how they value employees generally.
It almost certainly can't hurt: see FAQ.