Season 2 begins with Manson and his creepy-crawly crew in the midst of killing actress Sharon Tate and others in 1969 Los Angeles. The show then jumps back 18 months earlier, to show us the trail the cops are following, not to Manson, but to other cases, some of which will connect to the Manson “family.”
By showing us that things are slowly-but-surely headed towards their inevitable conclusion, it gives the viewer something to cling to, in knowing that yes, we'll get there soon enough if we're patient.
The notion of series creator John McNamara is to submerge you in the period and culture that gave birth to Manson and his small army of disciples willing to kill for him, and Aquarius does that very well.