I see comments about tax write offs like this, and I feel like either you don't understand what a tax write off is, or perhaps I don't. To me, spending $9 on s

I see comments about tax write offs like this, and I feel like either you don't understand what a tax write off is, or perhaps I don't. To me, spending $9 on something you are not using is always worse than paying your marginal tax rate on $9 and keeping the rest. Or am I missing something?