Why would you use a streamer like Wiim or Eversolo when you already have a streamer in Sonos?  And if you have a Wiim or Eversolo, why would you want to play it through Sonos?  It makes no sense.

Either way, if your Era 300’s are being used as surrounds, the Line-In is disabled.

Unless there is a note from the moderators, threads are only closed by time automatically. If there’s no indication from a moderator, it was closed because there was no activity in the thread. 

If you remove your 300 as a surround you can then use the line in.

Why anyone would choose to use an external streamer at high bit rates, convert to analog stereo and send that over a pair of RCA cables to a Sonos line-in is beyond my comprehension.

You have added unnecessary congestion to your network. Thrown away the benefit (if any) from the high bit rate source. Lost anything beyond 2.0 sound. Added a digital to analog conversion, followed by an analog to digital converson to further muddy the sound.

What am I not understanding here?

Leveraging the line-in input on a Sonos device enables integration with external audio platforms, allowing you to take advantage of their software features and processing while maintaining the existing Sonos ecosystem, thereby avoiding the expense of a full hardware replacement.

I sure hope that is an AI generated response, if not you missed a lot of the discussion's points.

Definitely AI generated.

This isn’t AI-generated. I only used AI to clean up my writing because my English isn’t great. I was responding to the point you made that I quoted. The Sonos software doesn’t suit everyone, and adding a dedicated streamer can solve this. 
 

Thanks for the reply, I'll try to be clearer here.

They are adding a streamer that duplicates functionality that already exists in the Sonos system. Maybe it has some features Sonos or the various supported streaming apps provide, I didn't look to see.

Adding a streaming device using the analog line-in option is the second lowest quality connection method possible, superior only to Bluetooth. Surrendering quality and multi-channel options for some control functions just doesn't seem a wise choice.

What makes you think Sonos is not a "dedicated streamer". 

I do not believe I stated that it was not.
 

Yes, I am not disputing that. I am simply suggesting that using an alternative device to interface with your music, while still relying on your existing Sonos infrastructure, could be a reason someone might choose to do this.
 

I interpreted adding "a" dedicated streamer as exactly that, rather than adding another or an additional one.