"Extended partitions" are really nothing special. Every disk has a partition-table, and the original design specified four slots. Therefore you could never have more than four partitions. Oops.
So, since it wasn't practical or possible to make the main partition table bigger than four-slots, "here's the hack they came up with."
That "fourth partition" could be subdivided into lots more so-called extended
partitions. This kept the original hardware table intact (with its four slots) and provided maneuvering room for expansion.
However, I think that the entire concept is much less important today than it used to be. Originally, partitions were mainly used as a different kind of "hack," to compensate for the fact that disk-drives were growing faster than filesystem formats. You had more capacity available than you could use
without partitioning it. This is no longer the case.