Hi,
Am using scan def for scan chain information, but i was having a doubt that why do we need to use scan def, as all the net connectivity is present in netlist and scan ports are already defined, so tool should be able to track throught the list easily. 
so can any one give examples of use of scandef,

Hi, 

As you have mentioned, SCANDEF has Scan chain connectivity information , which is also present in netlist. 

That make SCANDEF an optional one. Some of EDA tools, lets us auto- trace the scan chains ( without annotating with SCANDEF) , given the information on SCAN in/out points. 

However, there are additional information present in SCANDEF file, which makes it the easier way of mapping or annotating scan chains. For example, in case , when there are multi-input components in the scan chain, tracing scan chain becomes difficult. Also it is not possible to define PARTITION information about scan chains. ( Scan Flops can be swapped across chains with same PARTITION). 

These advantage of using SCANDEF, makes it preferred over auto tracing of the scan chains.

Hope this helps!!!
Daman