If you were credited with working so many days over a certain amount of time, the company was obligated to hire you, and start medical, pension and vacation credit, versus being a temporary we can let you go without a reason employee.
P.I.E. in Abq. at time was a breakbulk. When as a casual, if you got close to having the number of days worked where they were required per the Teamster contact to hire you, they would just stop calling you in.
Roadway Abq. in 1982 was working to many casuals to many hours and not following the contract. They were forced to put two casuals on full-time. One was me. Once put on full time you started receiving medical and vacation benefits. Your status was then unassigned. You were guaranteed forty hours a week but without a bid time. You had to be available for all the bid times.
In my I just want to be put on full-time world back then, I never heard of being able to combine your days worked at multiple companies to gain medical and vacation benefits.