I guess this can be useful if you don't know what's going on with your internet connection.

Although the fact that the "results" are cached arguably makes this less useful than GRC and other sites that will actively scan your IP (range).

And even though they provide a timestamp for the reported information (different for different ports by 24 hours or so, at least for me), I'd personally prefer an active scan over a database lookup if I want to know what's going on.

I'll go further and say that while the information provided is absolutely useful, depending on the services you're exposing to the Internet, there are other tools that will give you much more useful, actionable information.

There are rafts of online tools to check for vulnerabilities in specific services. Notably:

Mozilla Observatory https://observatory.mozilla.org/

Qualys Labs SSL test https://www.ssllabs.com/ssltest/

MX Toolbox Super Tool https://mxtoolbox.com/SuperTool.aspx

And many others, including the GRC Shields Up! port scanner mentioned in other comments in this thread: https://www.grc.com/shieldsup

As such, unless you're going to use Shodan services, or want to know what information they have about you, it seems like there are other, better tools out there.

What's more there are tons of tools that you can run locally that will provide much more information about the devices on your internal network, since you can run them inside your firewall.

N.B.: I am emphatically not discouraging others from using shodan.io, nor am I claiming that it's bad. Rather, I'm expressing my own opinion as to how I prefer to identify and test my internet-facing attack surface.