Enforce column uniqueness   Without an index, SQL Anywhere has to scan the entire table every time that a value is inserted to ensure that it is unique. For this reason, SQL Anywhere automatically builds an index on every column with a uniqueness constraint.

Reduce locks   Indexes reduce the number of rows and pages that must be locked during inserts, updates, and deletes. This reduction is a result of the ordering that indexes impose on a table.

Estimate selectivity   Because an index is ordered, the optimizer can estimate the percentage of values that satisfy a given query by scanning the upper levels of the index. This action is called a partial index scan.