One of many cool features in SLA is creating manual journal entries. Many times we have a need to pass manual entries to reverse the entries happened a source transaction because the account is wrong or date is wrong or even the value is wrong. All sub ledgers give you a way to reverse the transaction and hence reverse the associated accounting entry. But sometimes you may run out of time and luck and resort to manual entries in GL. Once we are GL to enter manual entries for transactions that pertain to sub ledger, we lose an opportunity to reconcile as we do not have primary references for which we have created this accounting entry for. If we have any reports that are based on SLA tables the reconciliation becomes harder as we cannot link anything that happened in the GL with the SLA. After all, the purpose of SLA is to eliminate this risk of reconciliation with GL.
Also, I guess, Oracle Support uses this route (manual journal entries) to fix any entries that were corrupted in the original transaction.
While entering these manual entries we can capture original transaction attributes using supporting references feature where you can capture attributes like asset_id if it is Assets related entry, transaction_id if it is related to Inventory, invoice_Id for AP and customer_trx_id or cash_receipt_id if it is AR invoice or cash respectively, These give meaning to the manual journal entries.
Process comes in three steps: 
Creating entries, saving them as incomplete entries with in SLA to be posted later and finally completing this so that this will automatically transfer to GL, journal Import and Post them as needed. To create manual entries you need to assign a sub-function SLA: Create Subledger Journal Entry to the menu. It will appear as a button in the view journal entries page.
The same can be achieved with an API XLA_JOURNAL_ENTRIES_PUB_PKG. Also this is immensely useful if you want to create accounting entries related to transactions in legacy or related systems. You can capture the supporting references related to the related system for reconciliation purpose.
The best part of using this API is that, we do not have to separately code for Journal Import and Posting (with complete journals parameter to complete, transfer and post). The value you pass to this parameter takes care of that.
Sources:Oracle Financial Accounting Hub Implementation Guide (Chapter 12 on Manual Sub ledger Journal Entries API) and Oracle Subledger Accounting Implementation Guide (Chapter 6 Sub ledger Journal Entries).