Since it comes up pretty often, and I'm not sure it's common knowledge, but writing things off really doesn't save you that much money, and in Phil's case does him even less good. It does save some money on your tax bill, but even writing off 20,000 dollars of his income as business expenses will leave him 100k to pay on. Add on top of that the fact that he saves literally zero dollars, and his bill is still going to be ridiculous. He was already on 2 seperate back tax payment plans when the bank leaks happened, and I guarantee he is just planning to do the same thing every year.
Attempting to write things off is just his Phil-style way of telling himself that he's doing something about his looming IRS issues while still spending every available dollar on retarded addictions. I've always assumed that it's a big part of his extreme anxiety about maintaining his non-goal minimums per stream. He already owes all of that money to the tax man, and if he ever stops making money even for a little while, that debt is still there.