Any improvement for partition table on same segment?

For large table, if want to partition the table, should use different device for different segment, then improve IO performance.

My question is: is a large table mytab has existing data on segment mysegment. Then running following:

ALTER TABLE dbo.mytab PARTITION 3

go

This is the simplest way for partition. but for this case, the 3 partitions will be on same segment? If so, any performance improvement for this case?