I have heard this exact advice all my life, but to me it’s not always realistic…. Personally speaking, I have done it (quit jobs without having something else lined up) and it’s usually turned out fine.. ive also had it not work out fine and really get me stressed out and have to take a job i really didnt want until that next thing came along…. The thing is, it’s REALLY hard to devote the time you need to applying and interviewing for new jobs if you already have a fulltime job… like, so hard that im not sure how people do it…. Whereas, if im unemployed, i can, and do, always treat the new job search/hunt like a fulltime job in itself, which puts me at a distinct advantage .