If you're at this point in the process, you have a design that's basically done. You've crossed the t's and dotted the i's, potentially created animations and interactions, and applied visual design to your work. But, maybe you still have a few questions about the usability of the product, especially with color and interactions applied. Or maybe you just want to have one final gut check of your designs.
If you feel the need, then conducting a "final" round of usability testing is probably a good idea. This is an optional step. Choosing to run usability testing at this point is entirely dependent on your project's needs, goals, stakeholders, etc.
P.S. The reason "final" is in quotes is two fold:
1. You might discover from this round of usability testing that you need to change some things and test again. Hence, not final.
2. The UX process is iterative, so while this might be the final test for this specific design, you might be back to improve what you just designed later on.
