Why do we have to pay a transaction fee to join the channels? Look at the picture. There are two  tokens in this picture .
Cof :When you join the channel, an NFT is created with information specific to you and sent to your wallet .
Pm :
When you stake your alfa in a channel, an NFT is created with information specific.

In fact, you are paying a fee of mint . 
An instant payment protocol, meaning you don't need to pay for a lot of timeshare and you'll pay as much as you use the channel.
This can be very useful for some shared applications such as vpns and games and applications