Iride01 summed it up pretty nicely, but I’ll add, addressing the question in the OP, that 8spd is less desired because it is older tech and because it delivers fewer gearing options than 9, 10, 11, 12, or 13spd cassette systems.
As noted upthread, buy as many gears as you can afford, because it’s good to have options...especially when you don’t know exactly what you need or want.