In order to effectively use a multi-core CPU, a decomposition of code into tasks must be made. By decomposing code into tasks, the tasks can be distributed among CPU cores by a scheduler. Tasks can have dependencies on other tasks, meaning that the tasks execute in a partial order. This partial order can be represented as a graph, where each task has predecessors and successors. A task can only execute when all its predecessors have completed.
To implement this, we can explicitly create a graph of tasks, with edges between tasks to identify predecessor/successor relationships. The problem with a general task graph system is that it's not productive to work with. Maybe it's okay for some purposes to have a GUI interface to build a task graph, but that's too tedious for everyday code. Imagine if you had to explicitly create nodes and edges when implementing a single-threaded function call graph, it would be so tedious!