There still is some buying and selling to do in a passively-managed fund. The stocks might pay dividends. If the fund manager didn't reinvest these dividends, the fund would begin to accumulate a cash position, which would cause it to stray from being an index fund. Stocks come and go from an index as well; if the fund is to maintain a composition that matches a particular index, this must be taken into account as well.
The role of the manager is to ensure that the fund maintains the composition that it was intended to replicate. It doesn't involve as much "stock picking" that active managers do. The manager has less leeway as to what s/he buys and sells, but there still is work involved.