Personalization changes your own working view. Choosing list columns or saving a personal favorite normally helps you without redesigning the instance for everyone. Configuration changes can alter forms, fields, and behavior for other users, so they need testing and a migration plan.
An application or plugin can add capabilities and dependencies. Activation is an administrative decision, not a personal display preference. Confirm entitlement, dependencies, and the target environment before activating one. The simulator teaches that decision through scenarios because it can't install hosted ServiceNow software. A blank application menu may reflect a role or a missing capability. Changing your theme will not grant access. Keep those causes separate before you request more privilege or activate a feature.