One benefit of Automatic Segment Space Management is the number of bitmap freelists that are guaranteed to reduce buffer busy waits.
Prior to Oracle9i, buffer busy waits were a major issue for systems with high concurrent inserts.
As a review, a buffer busy wait often occurs when a data block is inside the data buffer cache, but it is unavailable because it is locked by another DML transaction.
Without multiple freelists, every Oracle table and index has a single data block at the head of the table to manage the free block for the object.
After any SQL insert runs, it has to go to this segment header block and get a free data block on which to place its row.
Oracle's ASSM feature claims to improve the performance of concurrent DML operations significantly since different parts of the bitmap can be used,
simultaneously eliminating serialization for free block lookups.