If you set the countdown to 10,000 ms then the task will usually run in about 10 seconds, but sometimes the task can be delayed by a few minutes. They are intended for tasks that the end user isn't waiting for.
Relative times would work if all processing was handled by a single server with perfect availability. Instead we have many servers, each with imperfect availability, so we add tasks with absolute time, knowing that clock skew is usually small.