Cacher is the code snippet organizer for pro developers

1) add_image_size() doesn't generate the thumbnail, it just registers an image size as available to WordPress. so if you are using many different size of thumbnails on homepage, using add_image_size will generate a new registered size image with each post's featured image. Therefore, there will be so many images created on server. Sometimes, not all post image need to resize, so unnecessary images.
2) so instead of create a new sized image for every image, we can resize image on the fly. So that only certain category images that required can be resized. earlier we use timthumb for this. But it has encounterd with some vulterabilities so can not be used. Other options are
3) Aqua Resizer  
4) vt_resizer (default version does not work with wp multisite, but modified scrips are available)