Not so much about managing your spending, but checking non regular payments.
With so much shopping being online and Merchant details not always identifying the website you purchased from, ensuring that purchased are legitimate and for the correct amount is important.
Before Snoop, when receiving the Credit Card statement (we make all non regular purchases this way) we would go through and check them, writing a note along side to explain charge (single transactions not from the regular places) or ticking off the numerous purchases from the likes of Amazon and eBay checking that it was received, and amount is correct.
The features to add:
Check box related to each transaction - labeled along the lines of verified, confirmed, or checked
Simple text field related to each transaction to allow a note to remind you what it was for. This would help when looking at spend analysis to quickly identify the reason. Also, as we have joint accounts, saves me pondering about a transaction, when it was partner who made it.
A more complex addition would be assigning/attaching a PDF/jpeg on a one-to-one basis or one-to-many. Useful for keeping the invoice or receipt, which could be to remind you of what it was, or more importantly if you need to go back to the merchant with an issue related to the purchase.