Why Do We Need To Collate One Column While Creating Table?

Hi,
Good morning to All.

I have some doubt.
I cannot do anything without knowing the reason why we are doing that particular thing.

Any way, my doubt is:

While creating table, in some places I see COLLATE clause.

For example, recently I have seen this.

CREATE TABLE dbo.OrderDetails
(
       OrderDetailID INT IDENTITY(1,1) NOT NULL,
       ItemNumber VARCHAR(20) COLLATE SQL_Latin1_General_CP1_CI_As NULL,
       Qty INT NULL
)ON [PRIMARY]

In the above table, why the Collate clause for ItemNumber column? What is the advantage with that clause?
I have seen sql server help for this. I have found 1011 results for the query SELECT * FROM fn_helpcollations().
Then I searched for SQL_Latin related collations using: SELECT * FROM fn_helpcollations() where name like 'SQL_Latin%'
I have found 31 results.
How can I know which clause i have to use for which type of query?

Can anybody please tell answers for all these doubts?

Thanks in advance.

Regards,
Ashok kumar.