When you choose the columns for a PI or PA, consider the nature of queries likely to be made against the table. Typically, when a query specifies the values for each of the indexed columns, query performance is faster than the same query if the columns are not indexed.
A PI or PA allows the database to identify the single AMP that stores the data of interest for the specified values of the index, without requiring a costly database search on all the AMPs that store the rows of the table (called a full-table scan or FTS). For a PI, the rows of interest can be more quickly found on that AMP because the hash value is used in the ordering of the rows on an AMP.