WHY PGA IS REQUIRED FOR EVERY USER ?

Question

Good Morning Everyone ;
I have a question  about PGA.
WHY  PGA IS REQUIRED FOR  EVERY USER ?
What i got from google ..
Even though the parse information for SQL or PL/SQL may already be available in library cache of shared pool,
the value upon which the user want to execute the select or update statement cannot be shared.
I cant realize it   Can anyone show clear  example , if  possible ?
DB Version is  10.2.0.4.0
OS : oracle linux 5.5
Thanks in advance ..

Answer 1

Thanks aman and heok.
My Question :
Your explanation is clear. I think i am getting little bit confused.
Could you please clarify little more ?
>> session 1 :
user is HR
SQL>select * from tab1  ORDER BY name;
>> session 2 :
user is scott
SQL>select * from tab1 where ORDER BY name;
>> session 3 :
user is USER1
SQL>select * from TAB1 where ORDER BY name;
>> session 4 :
user is USER2
SQL>select * from TAB1 where ORDER BY name;
IS this right aman ?
Already sql statements are avail in SGA ,Even though all above users needs same information.
Oracle does sorting operation in PGA. If PGA exceeds , oracle will use temporary tablespace .
Thanks heok and aman.