As a result employees can't just bank their vacation hours and get a big payday when they leave. It tends to only take a couple years to get up to the max, and you end up with "use it or loose it" pto.
From the business perspectice services rendered previously is "nothing", because you can't get any more value from that
Which is why employers almost always cap in some form or another; the exact mechanism is partially determined by state law.
I worked for a company once that, during a bad spell, eliminated an accrual cap to encourage people to bank vacation. The result was that a not small number of people who weren't really into taking vacation just let their balances balloon. The company eventually forced people to work down their balances.