Why Microsoft Introduced multiple editions instead of single edition ?

Microsoft introduced multiple editions to support various kind of business applications such as small , mediaum and large scale applications.

I have listed below about each edition

1. Enterprise Edition : 

     a. Used for large scale applications .
    b. Supports all the features like logshipping , db mirroring
         Clustering,on-line indexing , data partitions etc .
     c. It supports clustering upto a mazimum of 8 nodes .
     d. No limit for noof CPU's .
     e. No Limit for RAM & database size .

2. Standard Edition :

      a. Used for medium scale applications .
      b. Supports clustering upto a maximum of 2 nodes .
      c. Maximum no of CPU's is 4 .
      d. No limit for RAM and database size .

3. WorkGroup Edition :

       a. Used for small scale applications .
       b. Doesn't support clustering .
       c. Maximum no of CPU's is 2.
       d. No Limit for database size  and RAM size is 3.

4. Developer Edition :

       a. Supports all the feautures of enterprise edition but recommended            only for development and testing .

5. Express Edition :

       a. A freely downloadable with minimum feautures .
       b. Maximum number of CPU's is 1.
       c. Maximum RAM size is 1 GB.
       e. Maximum database size is 4 GB .