Because if your mobile device isn’t connected to your network, then anyone using your Sonos system would just get an error when attempting to use that playlist. So any roommate or spouse would constantly be frustrated, until you got home. 
 

It’s important to remember, the Sonos ecosystem lives on the speakers, and not in the controller on your phone, so that any controller that can connect will see the same information.  The controller app is only a remote control to access the data that lives on the speakers. It has effectively as much intelligence as any remote you have for your TV or Cable box.
 

As mentioned on your other thread, you can create playlists on your Android phone and play them from 'on this mobile device' (not from Sonos Playlists).

This is entirely logical and consistent. Playlists on the device are played from the device. Sonos Playlists, which 'belong' to the system, are made up of tracks stored on the system.

I am the only user of my Sonos 1, so there would not be an issue with my playlist.  One question is that since I play music from my phone, then why can't it access my playlists from my phone while it is attached? The playlist would not need to be downloaded to Sonos, just accessed.  No problem with my car connecting to my phone and playing my playlists.

This also brings up another question: how come their isn't a USB slot, or accept a micro SD card, on my Sonos 1 so I could add memory to it thus enabling my music to be stored on my Sonos?

These are good ideas but Sonos efforts imo are around voice integration and streaming so these types of features are probably never going to happen. 

So Sonos does not have an option for my particular situation. My phone has playlists that allow me to shuffle my music at the click of 1 button. This also carries over to my car. It also worked with my old Bose.  However, my home Sonos system will not cooperate. 

Luckily I got the Sonos 1, so I have Alexia. So my Sonos 1 will not be just a paper weight on my end table.

No, Sonos does not customize their software for individual circumstances.  They design software for the majority of users.

How many times do I have to say that you CAN play playlists on your mobile phone on Sonos.

Thank you John. How can I setup a playlist on my Android that will be recognized by Sonos?

What phone hsve you got and what music player do you use?

Please also confirm what you see if you tap Browse, then On this mobile device, then Playlists.

My phone is a Samsung Galaxy S10+.  Android version 10.

I have been using Rocket Player for music.  My phone also came with Play Music, (Google).  Both have playlists, and neither playlist showed up on the Sonos app.  I do not mind if I have to install another music player if that is what it takes for playlists to show up in the Sonos app.

Browse → on this mobile device → Playlists = No selections are available.

OK that's helpful. Try installing Samsung Music if it isn't on your phone already. It should index your music on installation. Create one playlist in that app and then look in the Sonos app.

Then please report back. 

That worked.  Thank you for all your help John.

That’s good news.  I’m not sure I fully understand what is going on here, but it feels like Sonos is looking for the playlists in a particular location and some music apps put them somewhere else.  Or maybe it is an issue of playlist file format.  It may be that storing the music somewhere else on the phone, or changing some setting in the music app, would allow you to use a different app.

But it sounds like you are happy with what you now have, which is great.

Again, thank you John.  You have been a tremendous help to me.  It seemed strange that Sonos would accept playlists from some devices and not others. I had 2 strikes, but your suggestion of installing Samsung Music and creating playlist from that app was a home run.