It’s mostly a relic from an older time, it can be useful for more traditional services and situations that struggle with sharing public IPs. In theory, things like multiple IP addresses (and IPv6’s near unlimited addresses) could be used to make things simpler – you don’t need reverse proxies and NAT and port forwarding (all of which were once viewed as excessive complexity if not outright ugly hacks instead of the virtual necessity they are today).

Each service would have its own dedicated public IP, you’d connect them up with IP routing the way the kernel gods intended, and everything would be straightforward, clear, and happy. If such a quantity of IPs were freely available, this would indeed be a simpler life in many ways. And yet it’s such a distant fantasy now that it’s understandable (though a little funny) to hear you describe it as “additional complexity” when, depending on how you look at it, the opposite is true…

From a modern perspective, you’re absolutely right. The tables have really been turned, we have taken the limitation of IP addresses in stride, we have built elaborate systems of tools and layers of abstraction that not only turn these IP-shortage lemons into lemonade, the way we’ve virtualized the connections through featureful and easily-configurable software layers like private IP ranges, IP masquerading, proxies and tunnels can be used to achieve immense flexibility and reliable security. Most software now natively supports handling multiple services on a single IP or even a single port, and in some cases it requires it. This was not always the case.

It’s sort of like the divide between hardware RAID and software RAID. Once upon a time, software RAID was slow, messy, confusing, unreliable, and distinctly inferior to “true” hardware RAID, which was plug-and-play with powerful configuration options. Nobody would willingly use software RAID if they had any other choice, the best RAID cards were sold for thousands of dollars and motherboards advertised how much hardware RAID they had built-in. But over time, as CPUs and software became faster and more powerful, the tide changed, and people started to realize that actually, hardware RAID was the one that left you tied to an expensive proprietary controller that could fail or become obsolete and leave your array a difficult to migrate or recover mess, whereas software RAID was reliable, predictable, upgradable, supporting a wide variety of disk types and layouts while still performing solidly and was generally far nicer to work with. It became the more common configuration, and found its way into almost every OS. You can now set up software RAID simply by clicking an option in a menu, even in Windows, and it basically works flawlessly without any additional thought.

Times change, we adapt to the technologies that are most common and that work the best in the situations we’re using them in, and we make them better until they’re not just a last resort anymore, but become a first choice, while the old way becomes a confusing anachronism. That’s what multiple public IPs have become nowadays, for most purposes.

The main thing is log separation

It’s a lot easier to figure out why your service is broken if it’s the only service on the IP versus there being 10 services on the same IP all dumping into the same log. Lets me grep a lot easier

If your vps is a firewall, you could use it as an exit point for different private networks: ip1 to mask the traffic for a guest subnet that you don’t trust and if the ip gets blacklisted there are no issues for lan traffic behind ip2 while ip3 is reserved for server traffic with specific rulesets on supplier’s systems for updates/backup/whatnot. Should you have more than one mail server because of reasons, if one is blacklisted the other could remain clean (in this situation you usually put them on different subnets but whatever).