Feature could be used either to create or update test plans indeed.
Generate test plans can be time consuming manually, whereas having everything ready in excel makes it very easy for reuse, with all executions planned and unexecuted (if unexecuted in excel), organized in cycle and folder hierarchy (either reused if existing else created).
Also during test execution it can happen that testers are external / have no access to JIRA, or have access but with connectivity issues (jira cloud is globally slow which includes zephyr for jira cloud + when your organization is spread around the globe it will be extremely slow for some people). In such cases, especially business users mobilized for UAT do not want to waste their time on a slow reporting interface and prefer work in excel, which I can understand. The ability to update zephyr/jira with their updated excel file at end of day would be amazing.
|As a...||QA Manager|
|I would like...||To import test results in Zephyr from Excel|
|So that I...||Can easily generate or update test plan with details coming from excel|