When it comes to manupulating and using remote web-site contents or working with urls curl url library is perhaps the most useful tool at hand of developers.It can be used from standalone applications running in server or even desktop to do site scrapping as well from live web pages.
It alone contributes a lot to make and increase the popularity of php It can be easily embeded within php pages and the functions are very user-friendly. You can say curl to be a site-scrappers tool. and most of the site-scrapping work in php or perl is done using curl in some way or other.
it can virtually do anythiing that the human interaction can do with your website :login in, store cookie callback header and write functions.....what else do you want .Yes java script support is not there. though it may be tricky to add this support in the main package. we can make add-on packages to include such support.