Why does there have to be a maximum average CPM for lines using the “Budget” and “Minimum CPM” strategies?

With the “Budget” and “Minimum CPM” strategies, the “Maximum average CPM per line” is used as the upper limit for CPM. When you use this indicator, impression purchasing and budget consumption proceed according to a planned schedule that keeps the cost to a minimum. The cost should not exceed the maximum average CPM per line. If the cost does exceed this limit, then bids will decrease in order to bring the cost back under the limit.