Some people don't like sequences on Oracle. Why? I think they are very easy to use and so nice. You can use them in select, inserts, update,...
I don't. I should point out that sometimes people hate what they don't understand.
Sequences are incredibly important for generating unique IDs. Conceptually, it's useful to have a method of generating an ID that does not depend on the contents of a table. You don't need to lock a table in order to generate a unique number.
Sequences can also be useful for generating keys across multiple tables that need to be unique. For instance, if I have a new item entering a system and I want to put a row in several tables at once, I can grab an ID from a sequence are reuse it when I insert into any number of tables. Done properly, I know the ID will not conflict with a value already in the tables and that each row will have the same ID.
I would assume these things are also possible with auto-increment columns as well.