Dear Adnan,

I imagine you will get more suggested answers to this question than you are likely to want to read :)

Here are a few to start with:

DLS is most certainly not the world's best sizing technique -- trust me.  But I think the three reasons above contribute to its use and popularity.

Best regards and good luck with reading the other responses to this question,

Mike

Pardon my sleepy typing this morning, Adnan!  In point (2) I should have said...

"...Many TEM and SEM workers seem to  frequently take only a few hundred particles from their images to build up their view of the sample distribution.  As such, their statistics across the entire population of nanoparticles are not as robust as those from DLS...."

Another significant advantage is the fact that DLS is a solution phase technique and may give you a better idea of the behaviour of the sample in a system. One major problem with electron microscopy is aggregation of sample upon drying, DLS can sometimes give better information.

Also from DLS one obtains hydrodynamic radius of a particle which may and often is different to the real size of the particles but is extremely important for such phenomena as diffusion of nanoparticles.

it is relatively less expensive compared to SEM and TEM. It gives information of particle size distribution and polydispersity index. some instruments give information both about particle size distribution and zeta potential like zetasizer