Not certain i understand the migration instructions docs

[t68]

Preparing to upgrade a j1.5 vm 1 site to j2.5 vm2 abd doing some reading on http://docs.virtuemart.net/tutorials/30-installation-migration-upgrade-vm-2/77-upgrading-vm1-to-vm2.html
But I'm not sure about this partin the manual:

"Use the "Remove Virtuemart Tables" to delete all user generated data, after that use one of the migrationtools."

Why should I delete before i run the migration tools?

Hope someone can shed some light.

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