custom profile creation instead of modifying parent profile

I'm pretty sure that it's because upgrades might include changes to the default profiles, and may cause issues. So if you use a custom profile - even if it's just a copy of the default - you minimize the risk of encountering issues in the even something does change.

Parent profiles are created with "best practice" recommended settings and provide a great starting point for minor modifications that may only need to affect some environments. If you start editing the parent profiles, things can very quickly get complicated since a) that parent profile ultimately affects every environment, and 2) you now have to be extra diligent to offset all of the intentionally different settings across multiple environments (that would otherwise be implied by the untouched default parent profile).