If you pay for an expense with pre-tax income (i.e., in December, assuming a calendar tax year), that reduces your tax liability on your profits for the year, for an effective discount of X% on that expense (where X is your effective tax rate).

If you buy that same expense with post-tax income (i.e., in January of Year 2), you don't get the effective discount, and you won't know until the end of Year 2 if it will reduce your Year 2 taxes. Also, you have to wait a year to get any tax benefit out of the expense.

The cost of waiting just a few weeks can be huge. This is why many companies (or corporate departments) choose to blow the remainders of their budgets rushing through projects at year end rather than just taking the time to do things properly after the holidays.