What is purpose of planning tests 

Planning is the activity where you decide your testing strategy, e.g.,  which requirements you want to validate, how would you validate them, will it manual or automated, the resource allocation, and the execution plan (whom and when will execute the tests). Depending on several factors, you may want to prioritize some of the tests, or give more focus on some requirements. 

Although you may plan tests in different ways, Xray v2 introduced the Test Plan issue specifically for this purpose and we do recommend you to use it.

Inside your Test Plan you can put the Tests you want to track for a given version and the Test Plan will show you the consolidated results for those Tests. In other words, it will present the latest status of each Test, independently of the number of testing iterations (i.e., Test Execution) you make with them.

Xray v3.0 gives you the option to use the hierarchical test organization, which you may also take advantage at Test Plan-level by using the Test Plan Board. In Overview of the Test Plan Board, you can find out how it extends the existing Test Plan concept by adding the ability to organize a Test Plan's Tests within folders.

Below you can have a sense of how Test plan fits in the process