You would want to delete it as it keeps adding to it as you open various applications. Deleting it (purging it, actually) could add performance to your system.The kernel cache is a repository of prelinked kernels that OS X maintains to ensure that new changes do not harm the system. Every time a new kext is added, a new prelinked kernel is created and added to the repository, allowing the system to failsafe.
2. Onyx gives option to delete it: why would one want to delete it?
Nick, it's under the Cleaning/System tab, and it does offer to "delete" it. That's in the ElCap version of Onyx.
Ah, I misread your comment to be that you couldn't find it, not that you wouldn't recommend it. I'm with you on that, if it ain't checked, you don't really need to clean it. The Onyx developer knows what is best. ;D