Tool Selection: System Maintenance, Cache Rebuild, Disk Permissions

Aside from those times when the system has gone wonky and some heavy or light duty maintenance has to be performed how often should those Tool Selections note above be done ?

Also, why does TechTool Pro 9.5.1 provide Disk Permissions when Apple's Disk Utilities for macOS Sierra does not provide or call for Disk Permissions to be performed ? Or, on the other hand, should Disk Permissions not be performered in macOS Sierra ?

( My current OS for an external FireWire HD : macOS Sierra 10.12.5
( Main HD : iMac 2009 edition ( 12 GB RAM )