Hello MadhuSudan,
Unique Data Records with the help of the Key Field or combination of them (as per design of DSO) helps to sort of the duplication of Data. Here the system ensures on the Uniqueness of Data.
And as Shadab said , there are certain scenario where we need to maintain the unique records like counter Id generation where it never re appears again.
You can take the example of Product Item generated in that case.
Secondly, checking this flag improves performances where system eliminates internal processes , knowing all data is unique. You can imagine or visualize the same , if there is no uniqueness of data, system needs to keep track of all the records in the Data Packet, a bit of overhead.
For Line Item Dimension : This means the dimension contains onlyhone characteristic. Here the system does not create a dimension table.. Instead, the SID table of the characteristic takes on the role of dimension table.
Refer the below document for more on LID
Hope this helps..
Regards
YN