Why is there an inherent conflict between planning and control uses of budgets?

The main conflict that results between planning and control use of budgets is that managers might place their own personal interests before the interests of the company. This might result in budgets that are easily achievable (resulting in bonuses) or shifting income from one period to another in order to achieve certain budgets that will result in bonuses.

I will use a real life example that happened to me to explain this. I worked as a B2B sales representative for a large corporation (we were only 2 B2B salespeople + 1 manager) and when sales were slowing down, upper management would set up bonuses for achieving certain sales goals. The problem was that intentionally certain large sales that required management's approval were delayed and total sales would fall. Then suddenly the bonus show sup and all the large sales were approved and in two weeks the sales goals were achieved. Since B2B sales are not about selling to a lot of customers, but instead selling to the right customers a lot of products, a couple of delayed big sales made a huge difference and a 1% bonus meant changing your old car for a new one.