Why do most PE buyouts use a locked-box completion mechanism, and what protects the buyer between the locked-box date and closing?

A locked-box fixes the equity price off a recent, agreed balance sheet (the locked-box date), so there's no post-completion price true-up — attractive in auctions because it gives price certainty, speed, and avoids drawn-out completion-accounts disputes. The seller bears risk before the locked-box date; from that date the economic risk and reward pass to the buyer, so the seller runs the business 'for the buyer's account'. The buyer is protected mainly by a leakage covenant: the seller indemnifies the buyer euro-for-euro for any value that 'leaks' out to the seller or connected parties between the locked-box date and closing — dividends, management fees, non-arm's-length payments — while 'permitted leakage' (agreed items like ordinary salaries) is carved out. The buyer therefore diligences the locked-box accounts carefully and defines leakage tightly, since there's no later adjustment to catch problems.