it was beneficial for them because i was able to offboard and knowledge transfer much more effectively over a few months of part-time work as opposed to the standard 2 weeks notice.
it was beneficial for me because right out of the gate i had a consulting contract with a yc startup at a rate that was slightly higher than my yearly salary which i could use as social proof when reaching out to new clients and negotiating rates.
i ended up consulting for about a year as my only source of income where i took on a couple other clients through my network (ex-coworkers starting companies and needing help faster than they could hire, ex-coworkers referring me to others who were stating companies) and a few lucky connections through upwork (he contacted me on linkedin so upwork never got a cut of the invoices i sent). i've also tried toptal and moonlight but no contracts have come out of those platforms.
a year ago i took on a new full time job and i've been consulting on the side since. i agree with ananonymoususer in that i can have as much extra work as i can handle however i was lucky to have already created relationships with clients before getting a full time job. i'd imagine approaching a new client while maintaining a full time job is a harder sell than simply letting an existing client know that your availability will be reduced moving forward.