If I have remembered it correctly the Hohenzollerns tried several times to annex the electorate/kingdom of Saxony.
From the reign of Frederick II (the great) to the aftermath of Napoleonic wars where Prussia gained part of Saxony, and at the end of Austro-Prussian war where they planned to annex the entirety of Saxony which according to memory was only prevented due to interference from Napoleon III.
The sources are from Wikipedia which usually just consisted of few words:
As for the question itself: what made Saxony so valuable? Was it the natural border with Bohemia, or were there any mining resources like in Silesia, or was it simply because it was the nearest unconquered land?