Final answer:
Management accountants play a vital role in business planning by understanding the success factors that create value, which aids in attracting investors.
Step-by-step explanation:
Management accountants serve as key business partners in the planning process because they understand the key success factors that create value. This knowledge is essential because as a firm becomes more established, and its strategy is likely to lead to profits, the availability of information about the company's products, revenues, costs, and profits becomes more widely available.
Consequently, outside investors like bondholders and shareholders, who may not know the managers personally, are more inclined to provide financial capital to a firm with transparent and promising financial prospects of company.