Why using image resizing scripts in WordPress?

There have been a few threads about Timthumb and other similar scripts to use in themes for the image resizing. I’m very interested to know that why to use these scripts and why not simply use WordPress built in image size function. What advantages these script offer?

Since these scripts also create an additional copy of image on the server, so it does not decrease the server load in any way.

WordPress resizes only on image upload.

TimThumb and those other scripts resize when requested.

now my question: Is it wise to resize images based o request? what if you have like 1000 visors within one minute, won’t that be a problem for your server’s computation resources?

jayc

you resize the image once. on the next request you check if the resized image exists

PixelBook said
you resize the image once. on the next request you check if the resized image exists

this