Can someone explain the advantages to me?

If you want to be able to play music in perfect sync across more than one room/zone, then yes, no one does it as well as Sonos even at this time. 

If not, yes, Echo Dots using an audio out is a more flexible solution in that the end speaker can be at any price point that makes sense from a value for money/sound quality perspective - from USD 100 to USD 10000 or more.

The other thing that Sonos does better than most - certainly slicker - is playing music from music files on a local NAS. Again, with the advent of streaming services this is becoming a niche interest. 

Personally I find the Audio quality of Sonos to be much better than the other offerings, also the software to control the speakers is easy and it works. There are also many more music services available.

As for using the Google Home app… don’t get me started on that pile of …..

Another advantage of not using Sonos is using something like using an Echo Show 5, with audio out supplying signal to speakers of choice. That allows album art to be seen for music from streaming services. Music play can be via voice or phone casting for services like Amazon Music or Spotify. Sound quality will be as good or as bad as what the chosen downstream third party speaker provides. 

The other advantage of the non Sonos route is that the smart front end can be replaced quite cheaply as and when it obsoletes, leaving the investment in the downstream, non Sonos kit, protected. Allowing that investment to be higher if that is needed to obtain better quality than what Sonos and others at the Sonos price point will provide. This investment can then be in higher sound quality speakers, that do not have any smarts built into them, but which can be used for many generations of smart front ends.

But if one wants the music to be played at the same time in a quick and reliable way to other music zones in the home, then the downstream unit needs to be a line in jack equipped Sonos, as do the speakers in the other rooms. If not, Sonos isn't a must.

Audio quality is far better than Echo Dots, and you can group and ungroup rooms on the fly, something you cannot do with Echo or Chromecast.  Simply put, Sonos’ multi-room functions are far superior to anything you list. 

One other feature that’s been hinted at, but not outright mentioned is that Amazon and Google speaker only allow you play streaming sources.  There is no TV audio or aux line in input that can then be shared throughout your home, like you can with Sonos.  

You can get 3rd party soundbars and such that can do Alexa or Google Home streaming, and play TV or aux audio, but you cannot share that audio with other speakers in your home.

Amazon will play music from a local nas with full voice control, via the my media for Alexa app, in addition to streaming services. Again, with album art for screen equipped echos.

But yes, if something like a turntable is to be heard around the home, then a line in equipped Sonos system is required. 

Could you then play the local media on multiple echos in sync? I could be wrong, but I believe the answer is no.  I took a quick glance at this and it appears to be a normal Alexa skill, rather than a supported audio streaming service. 

No, multiple echos are not supported at this time, but are promised as a future development. But I am not convinced about echos for multi room from any source, in the manner they do today. If perfect sync multi room is needed, as I have written already on this thread, Sonos is probably the best solution at this time. Perhaps Bluesound does this as well, but I don't see it as value for money compared to Sonos for any kind of solution. 

• Wide range of audio device options and configurations for different applications
• Generally very good audio quality across the range
• Sophisticated (and patented) inter-speaker mesh networking
• Open control plane (using UPnP) for third-party control applications / automation (at least for the time being, since it’s not officially supported)
• Large ecosystem of music service integrations

And a rather strong support network in these forums, beyond the Sonos folks.