Struggling with Control Accounts

Hi all,
I was hoping that someone may be able to impart some wisdom, or a mnemonic or anything, really, to do with Control Accounts.

Here's what I know so far:
- Control accounts are summaries of accounts
- If there's a sales ledger control account then the sales ledger is not part of the double entry
- Sales from the sales day book are input in the debits column of the sales ledger control account
- Sales from the sales day book are still split out and documented in the sales account and the VAT account

Here's what I'm finding hard:
- How does the subsidiary sales ledger relate to this?
- What really is the point of a sales ledger control account, aside from 'summarising'? In the examples that I've seen, accounts that do not have a sales ledger control account simply put the total of the sales day book into the sales account. So what's the point of putting it in an entirely separate book?

I know I'm probably overthinking this but it feels like I've been staring at this concept for ages and it's started to lose what little meaning it had!

Any help would be much appreciated.
Thanks