Honestly given current technology, it makes the most sense to care for the Earth than attempt to colonize other planets, which are fundamentally unlivable in a myriad of ways. Survival of our species doesn’t mean a whole lot if it is 8 guys in a Mars base dependent on regular supply missions from Earth.
Not to mention, if we do focus on our planet and how to best manage the environment, it teaches lessons on how to potentially manage other planets’ one day.
The context for that number specifically is this Kurzgesagt video, which suggests that calendars should set their year zero to 12 000 BCE, when human civilization first began.