If I spend £1200 on a computer, that
full amount is deducted off my annual profits. I pay tax on profits (after I reach my personal tax allowance). I buy a cheaper computer and it translates to making more profit, which in turn means paying more tax. I'd rather have a more expensive computer and pay less tax, than have a cheaper computer and pay more tax. It's not necessarily about saving money using the tax deduction, it's about me spending my money on me and not donating it to the government.
Does that make more sense?
But again, assuming both computers would do the task equally well, you'd have more of your own money to spend the way you want by buying the less expensive machine. Let's look at some (extremely simplified) numbers.
There are two computers which will do your job, one costs $750, one costs $500. During the year, your income is $2000, and your tax rate is 15%.
If you buy the less expensive computer, your income before taxes is $1500, and you pay $225 in tax, leaving you with $1275 net income to spend the way you want.
If you buy the more expensive computer, your income before taxes is only $1250, so you pay only $187.50 in taxes. Is this better? No, because you spent the additional $250 on the computer, your net income is only $1062.50.
You're down $212.50 compared to buying the less expensive box, and have *less* money to spend on what you want, despite having paid less in tax.
This is because of exactly what John Albert said, the "tax write off" only saves you the amount of tax you pay on the expenditure, not the full cost. So you spent $250 more, and only saved 15% of the $250 in taxes.
Now there are tons of details: tax brackets, capital expenditure as opposed to an expensed item, personal property taxes, etc. But in general, the tax argument really just doesn't apply here as a justification for the more expensive machine.