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Every transaction is recorded twice: as a debit in one account and a credit in another, equal in amount. Buying stock on credit increases stock and increases what you owe.
Because every entry has two sides, the totals must agree. That constraint is not bureaucracy — it is the error-detection mechanism.
A single list of money in and money out tells you your bank balance and almost nothing else. It cannot tell you what you are owed, what you owe, what you hold in stock or what you own.
Double entry produces a balance sheet as a by-product, which is the difference between knowing your cash position and knowing your financial position.
Rarely. Software presents invoices and payments; the entries are posted underneath.
No. Both sides can be posted to wrong accounts and still balance.
Only for the very smallest operations, and it cannot produce a balance sheet.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.