I think a part of this is fashion for wide bars at the moment and a part of it is about the width of your shoulders. me I am fairly narrow shouldered so run 660 on the solo and 685 on the tandem where the greater leverage is a help. I find the 660 more comfortable and will go back to that for touring teh tandem
Started riding back in 1994 and used what my bike came with, which back then as was the norm, was 560mm flat bars with bar ends fitted… So not very wide! Still, quite a bit wider than the bars on my mates bike at the time, which had been trimmed to sub 500mm when he inherited it!
Use 685mm or 710mm wide bars now, have tried riding bars wider but my shoulders just ain't that wide so it's more of a hindrance than a help.