Because some portion of the RAM used by your daemons isn't actually being accessed, and using that RAM to store file cache is actually a better use than storing idle memory. The old rule about "as much swap as main memory" definitely doesn't hold any more, but a few GB to store unneeded wired memory to dedicate more room to file cache is still useful.
As a small example from a default Ubuntu installation, "unattended-upgrades" is holding 22MB of RSS, and will not impact system performance at all if it spends next week swapped out. Bigger examples can be found in monolithic services where you don't use some of the features but still have to wire them into RAM. You can page those inactive sections of the individual process into swap, and never notice.
There is absolutely no point to doing that, which is why file cache is never swapped out. The swapped part is not-recently-used, wired memory from processes, so that there is more room for file cache.
Like a highway brake failure ramp, you have room for handling failures gentler. So services don't just get outright killed. If you monitor your swap usage, any usage of swap gives you early warning that your services require more memory already.
Gives you some time to upgrade, or tune services before it goes ka-boom.
If your memory usage is creeping up, the way you'll find out that you need more memory is by monitoring memory usage via the same mechanisms you'd hypothetically use to monitor your swap usage.
If your memory usage spikes suddenly, a nominal amount of swap isn't stopping anything from getting killed; you're at best buying yourself a few seconds, so unless you spend your time just staring at the server, it'll be dead anyways.
