Largely because of mobile phones, nicely developed in their own way in Japan, people often take photos with their mobile phones and upload them to Twitter and other SNS.
I'm not exactly sure but probably people are doing it to share the experience through photos with their friends.
And it is one thing I almost never do.
I simply forget to take photos of the food before eating it.
Here is an example; an empty glass. I had great pomegranate juice, and after drinking it up, I noticed the glass was so pretty.
My Japanese friends will wonder why I didn't take some photos of the juice before drinking it.