Some forums do both at the same time, which can be nice. E.g. showing every comment linearly with time, but each comment that has replies also has a "show repl

Some forums do both at the same time, which can be nice. E.g. showing every comment linearly with time, but each comment that has replies also has a "show replies" link which expands when clicked to show the replies as children. Those replies are the same comments shown later in the linear list; it's just a convenient alternate way of showing them on request in context.