VuGen provides the facility to use multithreading. This
enables more Vusers to be run per generator. If the Vuser is
run as a process, the same driver program is loaded into
memory for each Vuser, thus taking up a large amount of
memory. This limits the number of Vusers that can be run on
a single generator. If the Vuser is run as a thread, only
one instance of the driver program is loaded into memory for
the given number of Vusers (say 100). Each thread shares the
memory of the parent driver program, thus enabling more
Vusers to be run per generator.

We can use the memory effectively without any a wastage.

for example u started running a program than internally 
what happens, the program will be loaded into the memory
and os will start the execution of your program.

In the same time one of the running applications in your 
computer want to use the same program than what happens

if your program is process based
than a new copy of the program will be loaded into the 
memory and operating system will start the execution of the 
second one.that means there are two copies of the same 
program are there in the memory.here it is the case of only 
two if 100 applications want to use the same program 
simultaneously than 100 copies of the same program will be 
loaded into the memory.

if your program is thread based
in this case if the program is already loaded into the 
memory the os never loads one more copy into the memory.
the os executes the earlier loaded program.so if 100 
applications want to use the same program only one copy of 
the program need to be loaded into the memory.so there will 
be large amount of free memory.so u can load more number of 
vusers