Do people really get a lump of money just for agreeing to come work there - while they could just leave on day 2 (or day 1) and pocket the thing? Or - is it like a higher first salary? Also, if the position is highly sought-after - isn't it a bit weird that a signing bonus is offered?
Some signing bonuses help compensate for losing bonuses or unvested stock at a company you're leaving.
Signing bonuses are also great from a company's perspective, because they're a one-time payment that may get you to accept a lower salary. From the employee's perspective, they may be good or bad depending on your needs; consider carefully if you'd rather negotiate for a higher salary instead.
It's also been most common for new grads I think. Presumably, to help them get a jump start on student loans and get set up comfortably coming out of college, which is of course a bigger problem in the US than anywhere else.
But it seems to be becoming more common for mid-level and senior engineers as well, in these cases I think it's basically a loophole to make up for the fact that bay area engineering positions are continuing to get more competitive to fill, and for whatever reason companies don't fully want to acknowledge this and increase their official pay bands faster.
E.g. in the oil industry, a significant chunk of your compensation is usually a cash bonus. Equity is reserved for C-level.
The typical starting salary for a new grad PhD or MS in geoscience at the majors is around $110k/year. That usually comes with ~$15-20k signing bonus, that you have to pay back if you leave within two years. However, you don't get an annual bonus your first year. The bonus is ~15% at that level, so it works out to be the same as if you'd gotten your annual bonus your first year.
It helps with any incidental debt that was accruing (separate from student loans), as well as immediate expenses related to the lifestyle bump (e.g., move to an apartment that would pass inspection). Which also benefits the organization, by unburdening the hire of some of those distractions from work.
I suppose a requirement that the employee pay back the bonus if they leave within a certain period is additional incentive for the employee to be genuinely interested and committed, not intending to job-hop for the next slightly-better salary or more promising lottery ticket.
Also, academic research jobs should be high-pay! It's just that the unionization levels are too low.
In today's job market, in software engineering, it's the applicants that are highly sought after. So the signing bonus is used to make the offer more appealing in order to compete better for talent.
In a weaker market, signing bonuses could also be used as a consolation for a lower salary offer.