The message channels act as the first level of categorization/segregation of the query. You can have multiple topic-based threads depending on your business, for example, Payments, Pricing Queries, Returns, Feedback, etc.
The user can choose a particular channel and start a conversation. For example, on the Freshchat support, you can see that we have configured separate message channels for product queries, pricing, feedback, and support.
Creating multiple channels helps you route queries to specialized agent groups and to have context on the issue even before reading the query sent by the user in turn saves time and increases productivity.
The welcome message in the channel helps set expectations with the user - it can talk about what kind of queries will be answered and set up expectations of different response times based on the topic as well.
It also helps with reporting/analysis – one can compare and check how many messages were sent through a particular channel.
Since you can add channels on the fly, you can also add a new channel for an urgent issue about which a lot of customers are reaching out - so that the customers get to know you are aware and see your response to the situation even before they reach out to you. A good example can be a temporary channel enabled during an outage or when a payment method is failing.