Legacy spreadsheets often contain sensitive business data, so conversion should happen inside your controlled Windows environment.
The desktop converter processes files on your machine. That avoids the retention, data residency, and vendor review problems created by free online conversion tools.
Work from a copy of the source archive, write converted files to a separate destination, and keep logs that show what was converted and when.
Local conversion is especially important for payroll, pricing, customer lists, claims data, engineering assumptions, and confidential diligence materials.