From their FAQ

What is the point of Leta?

Leta aims to present a reliable and trustworthy way of searching privately on the internet.

However, Leta is useless as a service if you use the perfect non-logging VPN, a privacy focussed DNS service, a web browser that resists fingerprinting, and correlation attacks from global actors. Leta is also useless if your browser blocks all cookies, tracking pixels and other tracking technologies.

For most people Leta can be useful, as the above conditions cannot ever truly be met by systems that are available today.

If you use their VPN and their browser or any firefox properly configured there is no reason to use Leta.