The producers of Doctor Who thought they needed to show more about the mysterious new ship the TARDIS, and brought us the moody two-parter which starts tonight on Retro TV.
Not to mention that a story that takes place entirely in the TARDIS makes for a very cheap to produce adventure, and an easy way to fill out the show's initial thirteen episode order.
Of course by this point, Dalek-Mania had already seized Britain, and the show would go on to produce...well, more than thirteen episodes.
We learned a lot about the TARDIS in this episode, including her ability to protect itself from catastrophic events like being destroyed at the dawn of time. Years later in Logopolis, the ship would be sent back to the creation of the galaxy, as it had already been set in canon that the Big Bang was a no-no.