expression pick whip vs property pick whip

Hi,

I dnt understand the difference between expression pick whip vs property pick whip.  both write  the same  expression if  i whip preoprties of  one layer to another. 

example, two shape layers,  red square and  green square. if I whip scale of green to red square,  using 'property pick whip' it writes-  

thisComp.layer("Shape Layer 1").transform.scale

and if I pick whip sacle using expression pick whip it also writes- 

thisComp.layer("Shape Layer 1").transform.scale

😮 

why I need these two? they are doing the same thing  whats the real difference between theses two? Google fails me. only I get articles abt  parent pick whip.

There must be some main differences. what is that. please help me.