The idea would be that every employee starts at the same pay rate but the pay rate increases with each year the person is employed.
Say the entry pay is $17.
You work there doing the same amount of work as the person who has worked there for 3 years but that other employee makes $18.50 because they have been there long enough to experience 3 pay increases on their check.
While the bumps are not major (which is a major con for some), they are guaranteed and consistent so you know your presence there does add up to better pay in the long term. You're guaranteed to eventually get an hourly rate which you consider worth more of your time as long as you put the hours in.
The benefit of this is it rewards consistency and commitment. Many times people find out the new person is making more than they are and they get frustrated and/or quit. This system flips that in the reverse. Seniority breeds economic benefit.