The Pixel is a good phone to test the latest android features for development purposes. I would imagine to some degree they are trying to target developers interested in testing software by offering the ability to unlock and relock the bootloader. This fosters a vibrant developer community and encourages innovation. Certain things can be tested in an android emulator but it helps to have a real device to test as well.

Pixels often ship with hardware features that other phones later include. For example Pixel 8 was the first phone with hardware memory tagging extensions and if developers wanted to test that feature they would buy a Pixel first and then use that experience with the devices their company is manufacturing. Pixels are often released with new android versions that implement android features and APIs the way they were intended to work. There have been cases of OEMs releasing devices with broken implementations of standard android features.

Pixel was the first phone with Strongbox as well. Additionally, It was the first android phone with satellite connectivity.

It also attracts the segment of the market that just enjoys modifying their phones as well. So basically they are targeting the power user community and developers. Despite the Pixel having the ability to install custom verified boot keys and custom OSs, Google knows that very few users use those features so it does not cut into their Play Store and Play Services market share very much.