FIRST_VALUE and LAST_VALUE are especially valuable because they are often used as the baselines in calculations. For instance, with a partition holding sales data ordered by day, you may want to know how much the sales for each day were compared to the first sales day (FIRST_VALUE) for the period, or you may want to know, for a set of rows in increasing sales order, what the percentage size of each sale in the region was compared to the largest sale (LAST_VALUE) in the region.
IGNORE NULLS is particularly useful in populating an inventory table properly.
Selecting neither IGNORE NULLS or RESPECT NULLS is equivalent to selecting RESPECT NULLS.