Posting keys given morethan one because of 
Example posting Key 40 is Debit
        Posting Key 50 is credit
That why they given more then one

In SAP 40,50 using for General Ledger Posting
       70    using for Assets Posting
etc
etc

why we have more posting key means the posting entry data 
will sit in different Table.

because there are different mode of data entry available.

for each separate data entry T Code you might be required 
different posting keys in cases which are not GL

The Subledger accounts do not have field status group. So
they are differentiated mainly via posting keys. Therefore
they need many posting keys to identify the nature of the
trasaction. But with G/L transactions the differentiation is
mainly done through Field Status Group.therefore they need
only two posting keys, one for debit and one for credit.

These are special classification keys. Two character 
numerical key it controls the entry of line items. 
Posting key determines Account type; Debit/credit posting, 
Field status of transaction.