Xray's Test, Pre-Condition, Test Set, (Sub) Test Execution and Test Plan are all Jira issue types. That means that testing artifacts are also managed in the same way as any other issue type you may have (e.g. Bug, Story).
This is a core and unique characteristic of Xray that leverages the native features and brings them to your testing artifacts.
Let's see some characteristics related with Jira issues that you can take advantage of also for Xray issues.
Organization Custom Fields Workflows Permissions Agile / Scrum Boards Changes
Assignment & Accountability Work Logs Integration with other apps Search + JQL Linking
Comments Notifications CSV Import REST API Reports / Listings
Organization
Projects can be organized in different ways, giving you the flexibility to manage all artifacts within the scope of one project or to split them between different project.
More info on project organization capabilities here.
Who wins?
Jira admins Project admins
Custom Fields
You may add additional custom fields to any Xray issue and include them in the screens.
Custom fields, whenever used properly, give the flexibility to provide additional information in a structured way to issues, so they can easily be accessed, searched and included in reports or even in Agile cards for example.
Who wins?
Jira admins Project admins
Workflows
You can implementation workflows also for testing related artifacts (e.g. Tests, Test Executions) to have full control over the lifecycle of the test specification or execution or planning phases.
With workflows you can even restrict that requirements are closed if they aren't properly tested.
Who wins?
Jira admins Project admins
Permissions
Deal with permissions as you deal with permissions in general for any issue in Jira. This simplifies and centralizes user and permission management.
Make your testing related issues read-only as you do for any other issue type.
Who wins?
Jira admins
Agile / Scrum Boards
Xray issues can be included in Scrum boards along with information from QA, thus everyone involved in the project can have a clear view about the real status of the issues being handled in the scope of some sprint, not just from a development perspective but also from a quality perspective.
More info here.
Who wins?
Jira users
Changes
You may track history and changes on any Xray issue.
Get aware of the changes made to test cases, or in other Xray entities, and who made them.
Who wins?
Everyone
Assignment & Accountability
Xray issues can be assigned, watchers can be added to them.
Ensure issues get done by assigning them to users, no matter if you're dealing with the specification of a test case or the execution of a bunch of tests within a given Test Execution.
Who wins?
Everyone
Work Logs
Ability to add work logs, namely in Test Execution issues in order to log the time taken to run the related tests.
Work logs can also be added to other Xray issue types, as way to report time spent on them.
More info here.
Who wins?
Everyone
Integration with other apps
Since JIRA issues is a core JIRA concept/artifact, they're used as the basis for many add-ons. Since Xray also uses JIRA issues, then the integration with many existing add-ons is available out-of-the-box, without having the needed of have a special integration for that to happen.
Some examples are Xporter, Structure, R4J and many other ones.
Who wins?
Jira users
Search + JQL
Easily search and interact with Xray issues using JQL, custom fields and specific JQL functions (more than 20!).
By using JQL and dedicated JQL functions, you can leverage your existing Jira knowledge to search what you need, based on the fields relevant to you.
More info here.
Who wins?
Everyone
Linking
Relate Xray issues to other issues in your JIRA instance (e.g. Story, Bug, Task, etc), the Jira way.
This facilitates clear navigation between related information, so you can easily access it whenever you need it.
Who wins?
Jira users
Comments
Collaborate by leaving comments to other users in Xray issues.
Reference and involve people in the design of test cases or in the definition of a test plan. Make contributions easier, no matter in which testing phase you're currently working on.
Who wins?
Jira users
Notifications
Notify users using Notification Scheme, the Jira way of setting email notifications.
Users can get notified by email, the same way it happens for other issues, so they can be aware of testing related events relevant to them.
More info here.
Who wins?
Jira admins
CSV Import
Xray entities can be imported by CSV, the same way that you can do for importing JIRA issues in general.
Xray issue types have some specific custom fields that give you the ability to do additional operations during the import process.
More info here.
Who wins?
Jira admins
REST API
Use Jira's REST API, out-of-the-box, to query, create or update Xray issues.
A simple example of creation of test cases can be depicted here.
Who wins?
Everyone
Reports / Listings
Xray issues can easily be included in listings since they're issues like every other one. Xray provides also some custom fields for each issue type that can be used in order to provide some relevant information, in the context of the entity that is being shown (e.g. "requirement status" for showing the coverage status of a requirement, or the "Test Execution Status" for showing the progress of a Test Execution).
The "Filter Results" gadget can also list Xray issues in order to make listings of Tests, Test Sets, Tests Plan or requirements and include, for example, information about their status.
Who wins?
Everyone