It’s now possible to book Southwest flights through the Chase travel portal. It looks like you can only book Wanna Get Away and Wanna Get Away Plus and even then not all flights will show up on the portal. There are three main reasons you’d want to book via the travel portal vs transferring points to Southwest directly (keeping in mind Southwest points are worth ~1.4¢):
When you hold a Chase Sapphire Reserve points are worth 1.5¢ towards travel
You earn points when booking through the travel portal
The portal offers some additional insurance
All of that being said from my quick search Chase seemed to usually be more expensive than booking directly on cheap Wanna Get Away fares and those are the flights that I usually book with Southwest. If you hold the CSR it might be worth doing some price comparisons but in most cases it doesn’t seem worth it. You also lose some flexibility when not booking directly as you can’t easily change your ticket if the price of a flight drops to get a refund.
Hat tip to Parts_Unknown-