Having the short-term control of the firm’s assets, managers might be tempted to act in the manager’s short-term best interest instead of the shareholder’s long-term best interest.
Consumption of lavish perquisites is one example.
Outright stealing is another example.
Some Russian oil companies are known to sell oil to manager-owned trading companies at below market prices.
Even at that, they don’t always bother to collect the bills!