Historical context may help: Some of this confusion is not your fault. Only very recently has the primary application had this "most meaningful experience" information. Before, you listed your ECs, and that was it. For years, many secondaries included an essay directed towards this same sort of information - either directly or indirectly. For sake of consistency, politics, or other issues; many secondaries have not been updated to reflect this change (yet). Some school may eventually alter their secondary prompts to focus elsewhere. However, others may just keep these essays, even if the do not yet acknowledge that you have filled out information on your primary already.
Either way, during this time of transition this may seem redundant. Just try and do your best and expound upon what you have already written.