All building seats does is incrementally upgrade your stadium to the next lvl rather than all at once. It costs a fraction and takes less time to do 1/4 at a time but in the end it's about the same.

I only do it when I don't have anything else to build AND I don't have the cash to do a full upgrade. Or at the lower lvls I did when I just needed to reach the attendance limit real quick before building the other stuff.

I am lvl 5 and have a stadium that has more than double the seats that I can fill due to stupid lvl limits (I have enough fans to fill it). But the point was to get the highest lvl youth academy as quickly as I could. Have to get to 43,000 seat stadium plus all other lvl 7 facilities to get that.