It does allow for some coarse grained improvements for certain services, though.
You might write an smtp daemon that only delivers email on to another service via LMTP - thus without ability to write to (or read from, depending on configuration) the file system, for example.
Yes, you can accomplish this via containers, too - but it's nice to get as much out of process isolation as possible, not to mention it is likelier easier to test and debug the closer to the surface of the runtime limitations are implemented.