Google Analytics loses the link between a customer choosing a product, going through the checkout, and making a payment. Littledata fixes this by linking together the session that the customer had browsing the products with the actual payment.
With Shopify's default GA tracking, many transactions will be attributed to Direct even when your traffic by channel for show users coming from a range of marketing channels (organic search, email, etc). This looks like Direct traffic for the purchase event, but 99% of the time it wasn't actually Direct!
When you install the Littledata app, we link the sessions together automatically. This is done through Shopify's backend so that the transactions in Shopify will always match what you see in Google Analytics (or data destinations via Segment or Measurement Protocol).
Note that Littledata also fixes marketing attribution for recurring transactions (ecommerce subscriptions) if you're using an app like Recharge and have activated our Recharge connection.