When you leave a job before finding another, you risk spending more time on the sidelines than you originally bargained for.
For most jobs worth having, you must go through an interview process. Even if you acquire a verbal job offer within a day, it typically takes the HR department the balance of a week to finalize your paperwork. All in all, that amounts to at least an additional week on the sidelines (and therefore an additional week of lost income) for you if you're not working.
Furthermore, being employed improves your bargaining position when you acquire a job offer because you have an alternative (BATNA): if you're not satisfied with the job offer, you can retain your current job. On the other hand, if you're unemployed, you're more desperate, and therefore more willing to take a suboptimal offer because you need to take "something".
Overall, employed candidates are generally more desirable than unemployed candidates for two reasons:
- Retaining a job implies commitment and a work ethic. Employers do not want to hire candidates who will flee or quit when things become difficult.
- Being employed implies competence. Although one could argue that being employed doesn't actually attest to a candidate's competence, an unemployed candidate often must supply legitimate reasons for his/her resume gap, whereas an employed candidate does not.
People typically acquire and retain jobs for at least one of the following reasons:
- They need to earn an income.
- They want/need to stay busy.
- They want to achieve professional/career goals.
In most cases, sitting on the sidelines without a job will impair your ability to achieve these goals.