Not every subscriber on your email list is on the exact same journey when it comes to making a purchase from your brand. The conditional split feature in Klaviyo is helpful if you want to create a single flow but then curate different content for recipients based on what you already know about them (e.g. emails opened, location, past purchase history, etc.).
For example, in an Abandon Cart Flow, you can use a Conditional Split to branch your flow based on whether or not a subscriber has already used the promotion available in your email. You may want to customize your messaging content differently for the subscribers that have not converted from your abandon cart with a code-reminder email. Here is a helpful video that will walk you through creating this conditional split in Klaviyo.