I can't say why the OP is looking at 8, but I can say why I did. I built up my first touring bike in 2004. At that time both 8 and 9 were common. I chose 8 because I wanted the rear to stay in adjustment longer, the distance between the rear cogs is slightly greater. And, I am using a Sram 11/32 cassette. Both the 8 and 9 speed cassettes have the following cogs in common: 11, 12, 14, 16, 18, 21 and 32. The only difference between the two cassettes is that the 8 speed has a 26T cog while the 9 speed has a 24T and 28T cogs. I concluded that I would rarely be on the cogs that are bigger than 21, so I would rarely be on the part of the cassette where I would notice the additional cog.

Right now I regularly use three 8 speed bikes (all three use the same Sram 11/32 cassette, one 7 speed, one 6 speed, my trainer bike (indoors on the trainer) is an old mixte frame 6 speed, and a Rohloff bike. Every one of the bikes uses the same chains.