The idea of placing the entire modify keyword logic within a document lock is similar to the idea of locking a document as soon as the document when an edit keyword panel is displayed in a client. The idea is that the document is locked both while the edits are made as well as when the edits are being persisted into the database. That way, a change cannot be persisted by another user while you are making your edits, which would be a possible data loss scenario.
It is also easier to scope out your modify keyword logic, which is why the DocumentLock type implements the IDisposable interface and can thus be used within a using statement (or try/finally if you are so inclined).
