Because we don't/can't do perfect lock-downs -- everyone literally stays home for two-to-six weeks and doesn't move from their home -- you can really think of lock-downs as doing two things.
1. It effectively reduces your population.
Not everyone can stay home, because we still have stuff that has to get done, but say 80-90% of your population can stay home, because they're retired or they have a white collar job that allows them to work from home or because they're children and school is closed or because they've been fired because no one is buying what their job is selling.
This means that instead of 85% of your population needing to catch the disease for it to burn itself out, you're only waiting for it to go through 85% of 10% of your population before it burns itself out. (Unfortunately, because that 10% includes the people who take care of the 0.5% of the population that is most vulnerable, this still kind of sucks for that 0.5%, and you're really waiting for it to go through like 10.5% of your population.)
You'll still see increasing deaths during a lock-down until all of the grocery store workers and doctors and firefighters who are still pretty exposed out in the world catch it and some of them die. This also sucks and is kind of unfair, especially since some of the jobs which are pretty essential weren't particularly great before the pandemic.
2. It effectively reduces the R0 of the disease even among the essential personnel.
The R0 of a disease (last estimated at 5.7 for the coronavirus) is not constant, but depends on the conditions it is spreading in. Even among the essential personnel still out there risking exposure, the R0 is lower with everything shut down than it would be without the lock down, because the lock down still reduces the number of people each essential worker comes into contact with on a given day. Dropping off packages at a hundred doors a day puts you in marginal contact with a lot of people, but it's still less than if you did that and then went out to a bar and a restaurant and a movie afterwards.
Suppose the lock down is dropping the R0 among the active population from 5.7 to 2; that's still not great, but that means that instead of 85% of the 10% of the population still out in the world catching the disease, only 50% of the 10% will be expected to catch it. This is more awesome and less unfair, and also protects the vulnerable populations that come into contact with the essential workforce.
So our imperfect lock downs don't stop the disease cold in its tracks, but it does probably reduce the number of people we expect to catch it from 85% of the population to -- if we're lucky -- 5% of the population.