A hot summer blows past our quarterly estimate and we don't find out until the return is done
Without a mid-year check, a shop's first real look at how a strong AC season affected its tax bill is often the finished return the following March. By then the year is closed, so there's no way to fund a retirement plan, buy equipment before December 31, or send in a catch-up estimated payment to soften the penalty.
That turns tax season into pure reporting instead of a chance to actually change the outcome, and it means every option that existed back in July is gone by the time anyone looks at the numbers.
A liability discovered in April with no advance warning can force an owner to draw on a line of credit to cover the tax due on top of the current quarter's estimate, and by then the deadline to fund certain retirement plans for a deduction on last year's return has already passed for good.