Why do I need to resize before uploading?

Forgive me if this is a very obvious question but........I understand that Squarespace automatically saves every image to various different sizes and uses the best size depending on device etc. However, it also advises that photos are uploaded at a reduced size to help with page loading speed.

My question is why is this necessary if Squarespace resizes them anyway - why don't I just upload the largest size there is and let Squarespace do it's own resizing?

Thanks!