I can purchase new equipment for my business...write off the whole amount this year...and when I pay it off I can't write off the amount I pay in the successding years?
Not the full amount. You depreciate according o the IRS schedule. You'll take a percentage each year.
OP really needs an account.
Depreciation write offs are suppose to reflect the usage loss on systems used in operating your business. AKA your computer handles your inventory management, but as it gets older, it is no longer worth the same amount, so it should not be inventoried at the original cost. The depreciation in value, being a tax write off because it is the cost of doing business.
If your business depends on the written word, it won't be around long enough for this to be a factor.