A primary goal of improving performance with indexes is avoiding table scans. In a table scan, every page of the table must be read from disk.
A query searching for a unique value in a table that has 600 data pages requires 600 physical and logical reads. If an index points to the data value, the same query can be satisfied with 2 or 3 reads, a performance improvement of 200 to 300 percent.
On a system with a 12-ms. disk, this is a difference of several seconds compared to less than a second. Heavy disk I/O by a single query has a negative impact on overall throughput.