To expand on Jeremy's answer:

The reason you want to do this is that the ID is not always portable between databases and should not be relied on to locate a record, but is the way some buttons are identified in Odoo XML files once they are inside the database.

If you see this:

<button name="396" string="Convert to Opportunity" type="action" help="Convert to Opportunity" class="oe_highlight"/>

You can locate it the button using:

name="396"

... but it is safer to locate it with:

name="%(crm.action_crm_lead2opportunity_partner)d"

Hello

When the xml template will be parsed  by Odoo and inserted in the database (in table ir.ui.view); the %(xmlid)d will be replaced by the real id from this record.