It can vary by company and culture. Tech may be different, but in non-tech roles where equity isn't expected in most roles, signing bonuses can serve a function to nudge someone to leave a role they're comfortable in, or to make up for other aspects the candidate is losing out on by leaving at a certain time (e.g. annual bonus with 10a 10% of salary target pays out in March, but the new company wants you to start in January).