Sequence vs identity

I think you will find your answer here

Using the identity attribute for a column, you can easily generate auto-incrementing numbers (which as often used as a primary key). With Sequence, it will be a different object which you can attach to a table column while inserting. Unlike identity, the next number for the column value will be retrieved from memory rather than from the disk – this makes Sequence significantly faster than Identity. We will see this in coming examples.

And here:

Sequences: Sequences have been requested by the SQL Server community for years, and it's included in this release. Sequence is a user defined object that generates a sequence of a number. Here is an example using Sequence.

and here as well:

A SQL Server sequence object generates sequence of numbers just like an identity column in sql tables. But the advantage of sequence numbers is the sequence number object is not limited with single sql table.

and on msdn you can also read more about usage and why we need it (here):

A sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The sequence of numeric values is generated in an ascending or descending order at a defined interval and may cycle (repeat) as requested. Sequences, unlike identity columns, are not associated with tables. An application refers to a sequence object to receive its next value. The relationship between sequences and tables is controlled by the application. User applications can reference a sequence object and coordinate the values keys across multiple rows and tables.

A sequence is created independently of the tables by using the CREATE SEQUENCE statement. Options enable you to control the increment, maximum and minimum values, starting point, automatic restarting capability, and caching to improve performance. For information about the options, see CREATE SEQUENCE.

Unlike identity column values, which are generated when rows are inserted, an application can obtain the next sequence number before inserting the row by calling the NEXT VALUE FOR function. The sequence number is allocated when NEXT VALUE FOR is called even if the number is never inserted into a table. The NEXT VALUE FOR function can be used as the default value for a column in a table definition. Use sp_sequence_get_range to get a range of multiple sequence numbers at once.

A sequence can be defined as any integer data type. If the data type is not specified, a sequence defaults to bigint.

Sequence and identity both used to generate auto number but the major difference is Identity is a table dependant and Sequence is independent from table.

If you have a scenario where you need to maintain an auto number globally (in multiple tables), also you need to restart your interval after particular number and you need to cache it also for performance, here is the place where we need sequence and not identity.