Use of png images

I am new to graphics and svg so there may be something obvious
that I am missing.
The test suite, and many other examples, use png images. I am getting the 
impression that the primary use of svg is for filtering png. 
If this is not the case then I can't imagine why the test suite is not based
primarily on svg. (period). 

It's not that I'm not interested in png but is it married to svg? Does
png make a better bumpMap than svg?

Also, somewhat off topic, I find that many (most?) svg files are 99%
something called 'javascript' and a couple of lines of svg. Somtimes the
code mentions ECMAscript but it means javascript. I can easily
find a specification for ECMAscript but I can't find anything in the
svg specification that defines the location of the definitive 'javascript'
specification. 

Many thanks for any enlightenment.

Dave Biggar