"Multi-offer" channels allow you to attach your offer to a product sheet that has already been created by another seller or market place.
Therefore it is not necessary to create the product using the "product" data. You can distribute your products using just the "offer" data. Depending on the channel, either the EAN code or an ID unique to that market place may be necessary to attach to the existing product sheet.
Usually the product data (title, description,...) are chosen by the first merchant who creates the product on the channel. Other sellers who attach to that advert cannot change the values. Only the "offer data" will be taken into account (price, stock, ...).
In this way the channel can have one product sheet for one product, which is offered by several merchants.
Without this process the channel would create as many product sheets as there are ads for each merchant.