yesterday I've read a Porsche expertise about all tyres/rims-combination that would be "allowed" by Porsche

I've found following

if you use 185 / 70 R 15 or 195 / 70 R 15, you could mount 'em on following rims:

5,5 J x 15
6 J x 15
7 J x 15

so I wonder what are the advantages or disadvantages of what rim width???
I mean, technical speeking - not personal taste/optical

get the wheel that's closest in with to the tire
that way, tire will deflect less under load = more grip

I'm not sure if I got it right

the 7" rim should be 178,8 mm, right?
so, for both tyres the 7" rim would be ideal ...

but why do they list a 5,5" (=139,7 mm) and a 6" (152,4 mm) rim as "good / o.k."

70 series tires have a (relatively) big sidewall, so lots of deflection available and therefore it is possible to mount them to less ideal width rims