What purpose do these products serve?

https://www.sonos.com/en-us/shop/sonos-line-in-adapter

https://www.sonos.com/en-us/shop/sonos-combo-adapter

Or are you looking to have Sonos recognize any and every type of music transmission codec via USB-C and play them?

Well... the products you mention allow USB to jack, not usb to usb. 

iPhones, Android phones now have usb-c but rarely audio jack output. Macbooks, iMacs have usb-c. Why shall I have a usb-c to jack for then another jack to usb-c cable to get my phone (wire) connected to the Era100?

Liable to be an answer you’ll only get from an audio engineer, programmer, or Product Manager at Sonos, then, not something this community would be able to answer. 

Alright, but who shall I exactly ask ?

Is Airplay or Bluetooth an option? Most phones have Bluetooth, even ones without a USB-C connector.

I would perhaps start by speaking to Sonos Customer Support - here is the link to contact them…

https://support.sonos.com/s/contact

… maybe let us know what you find out.

Although I think it is unlikely that you’ll get an ‘answer’, as this might be proprietary business information. And as such, they’d likely be unwilling to share. 

If it was ‘easy’ or a ‘slam dunk’ issue, they’d likely have done so from the beginning. 

USB is master/slave. Sonos have clearly implemented the master bit, to enable the dongles.

Implementing the slave part would be extra work. Personally I’d really welcome it if an Era100 could be wired directly to my PC, but it’s still a development that would have to take its place in the commercial priority queue.  

I think a USB to USB connection might also give people the impression that they can wire audio matched to video without a delay.  That’s not the case.  Perhaps bluetooth and airplay are better for syncing audio with video.

BT and Airplay can sync by applying ‘back pressure’ and apps then delay the video to suit.

A USB connection per se has insignificant delay, so the stream could be handled across the system the same as Line-In.