Why are these nesting magpies building another nest
I found an article that explains secondary nests as backups in case of nest usurpation by other birds:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4858914/
This is for black sparrowhawks losing nests to Egyptian geese around Cape Town. They measured number of nests built per breeding pair of sparrowhawks and found it to correlate with number of Egyptian geese present and especially their nest usurpation behaviour.

That article mentions brood parasitism (e.g. cuckoos) as another possibility for other birds, and I've seen references to duplicate nests with eggs in each. ( https://www.tandfonline.com/doi/pdf/10.1071/MU939286a )