The mainline is an active branch, with regular drops of new and modified code. Keeping it healthy is important so that when people start new work, they are starting off a stable base. If it's healthy enough, you can also release code directly from mainline into production.

Despite the value of release branches, most of the best teams don't use this pattern for single-production products, because they don't need to. If the mainline is kept sufficiently healthy, then any commit to mainline can be released directly. In that case releases should be tagged with a publicly visible version and build number.

Release branches are a valuable tool when a team isn't able to keep their mainline in a healthy state.