when wil we use select 1 from empselect 2 from emp.....(In real time)

Actually the Index feature is meant for Order By clause. Example

SELECT CustomerId, ContactName FROM Customers
ORDER BY 2 ASC

The above will Order the results by ContactName as it is Index is 2

while the below one will Order the results by CustomerId

SELECT CustomerId, ContactName FROM Customers
ORDER BY 1 ASC