Having more seats gives you more latitude with the pricing to deal with promotion/demotion changes in fan demeanor and better/worse than expected runs.
For discussion, let's assume that the willingness to purchase seats is based on the total cost (i.e., you can sell up to $X in luxury boxes, whether it's a lot of cheaper ones or fewer more expensive ones). Let's say that for a given team, they can sell $4200 in luxury boxes, so that a team with 7 priced at $600 and a team with 3 priced at $1400 is exactly even. What happens if you're slightly off, and only $4195 is available? The unsold seat costs $600 in the first option and $1400 in the second scenario.
Having more of a range for pricing helps as well. If you had one seat to set at a given value, it would be really hard to optimize your income, since a miss would be disastrous (and thus, a downturn from a bad league result more crippling). And in the end, if you never expanded, you'd be limited to $131500 at maximum seat prices for the starting arena, of $139500 if you added 5 luxury boxes.