Distorted bass sounds awesome — greasy, gritty, garage-y, and punchy. But if you're not careful, distorted bass can get lost in the mix. This is because using distortion can increase compression (reducing the natural dynamics of your bass), cut some of the low end, and juice the midrange frequencies, putting the bass right in the zone of any distorted and fuzzed out guitars you have. Especially if some of those guitars are baritone or seven-string distorted guitars.
By blending in some of the clean, undistorted bass guitar along with the distorted version, you can get those dynamics and low frequencies back, without compromising the distorted tone. Dial in just enough clean lows that the overall bass sound still has the desired grind and grit in the mix, but you have back all the low end thump that the distortion took away. This way, you can achieve a gritty, greasy groove without compromising the bass guitar's powerful low end.
In computing, an abstraction layer, or abstraction level, is a way to hide the implementation details of deep functionality, allowing the separation to facilitate interoperability and platform independence. This is how peripherals can work with computers using different operating systems. For example, an audio interface can connect to the USB port of a computer running either macOS or Windows; the interface doesn't need to be re-coded to work with each operating system, instead the interface only needs to have a driver that can run on each operating system's abstraction layer.