Node groups are optional and they enhance the profiler service mostly. The redundancy of PSN availability is dictated by the Radius Server list order on the NAD.
https://community.cisco.com/t5/network-access-control/ise-node-groups/td-p/3514849

https://www.mbne.net/tech-notes/aaa-tacacs-radius

Explanation

Monitoring = provides advanced monitoring and troubleshooting tools that you can use to effectively manage your network and resources Policy Service = provides network access, posture, guest access, client provisioning, and profiling services.
This persona evaluates the policies and makes all the decisions.
Administration = manages all system-related configuration and configurations that relate to functionality such as authentication, authorization, auditing, and so on pxGrid = shares context-sensitive information from Cisco ISE to subscribers
https://www.cisco.com/c/en/us/td/docs/security/ise/1-4/admin_guide/b_ise_admin_guide_14/b_ise_admin_guide

Section: Web Auth and Guest Services
Explanation/Reference: https://www.cisco.com/c/en/us/td/docs/security/ise/1-3/admin_guide/b_ise_admin_guide_13/ b_ise_admin_guide_sample_chapter_01111.html