For every accounting document there is a corresponding entry in the General Ledger (this is the same with almost every ERP and Accounting System in use today).
- account.invoice - stores Invoices and Refund Invoices (credit notes) for Customers and Suppliers
account.invoice.line - stores the lines from an Invoice
account.voucher - stores Payments
- account.voucher.line - stores the lines from a Payment.
- account.move - stores Journal Entries
- account.move.line - stores the lines from a Journal Entry (Journal Items)
These are ALL important.
A business document is a historical representation of a transaction and the general ledger is the central repository (based on double entry bookkeeping) recording those transactions so the impact on your business can be measured.
I suggest reviewing some background Accounting information, like http://www.principlesofaccounting.com/