Use of $Projection Keyword and Filters in Association

Hi Experts,

I want to know the use of $projection  keyword. when should we use this keyword.

I want to know the difference of below two statements

business_partners[ bp_role ='01'].company_name as customer_name --

I know this statement is used to display the customer name whose Bp_role value is equal to 01

But I didn't understand the below statement

mard[1: matnr ='0000000010'].lgort

Can you also please tell me about use of associations. Since we can use inner joins in which business scenario we can use associations.

Thanks

Pawan Akella