Valve guide clearance

The different materials expand at different rates. If you have a liner that expands faster/more than the cast iron head it is mounted in, the hole will actually close up a little until the head catches up. We're talking small amounts, but...
Same reason that you need a more significant shrink fit when setting steel into aluminum than steel into steel. Aluminum expands more and your insert could come loose if the interferance fit isn't enough.