File sizes... and Versions
You must deliver images in different sizes
Security: you never show the original image on a web page because that reveals its location to hackers
Bandwidth and screen size: you deliver low resolution images to mobile devices, especially helpful over thin pipes.
The requirement to create versions necessitates your application to be able to resize images. Most conveniently this is done upon upload. See workflow later
See image processing applications for a list of the best third party providers
What's more, even if you don't use the gallery for display, you'll be able to deliver responsive content using the HTML picture tag and media queries on all of your webpages.