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. 

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.

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.

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. 

• 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

Yes, but a thought - the reason why Sonos needs more of such support is because it is, as is always pointed out here, a networked computer system that just happens to incorporate speakers. And the consequent need for this support is the price paid by even those that do not use Sonos’ multiroom groups for any of their listening - the aforesaid networked computer architecture is still present to support the unused multiroom groups feature for such users. Would such users then be better off using another system that does not do groups, or does not do them as well, but also needs far less support? My Sonos system is now as stable for music play as my Echos, but it took some work to get there. The Echos needed none of that work.

With reference to Sonos net, to my surprise, this is what Sonos Staff said in another thread a month ago:

Considering that we now no longer consider SonosNet a preference, but simply an alternative,

I suppose this is because home Wifi is much better now than it was when Sonos launched, with a more robust coverage possible across the home.

Also, Sonos architecture is designed for the original use case back in 2005 - of using a single local NAS to play different music supplied by it, in more than one room at the same/different time, and to also play the music in perfect sync in all rooms when needed.  

For a use case where streaming services with family plans have made a single local NAS as redundant as Sonos made CD players back in the day, and where multi room grouped music isn't a big need, the simpler architecture used for Amazon and Google home wifi kit where more coordination work is done in the cloud, automatically translates into lesser support challenges for the kit at home - with this kit not needing to be networked computers in constant communication with each other - as they are in the case of Sonos. 

PS: a copy of something another expert said here a month ago: any device which simply connects out to the internet asks a lot less of the local network than a system which depends heavily on device-to-device communications.

Device in the above case being the Amazon/Google kind of speakers.

Now he tells us...