Disallow Images Robots.txt Why ?

Whats the reason (if is still advised) to have images as disallowed in robots txt file, Im looking into getting my images more prominent in google image search and this is one of the questions that has always been in my head.

I have auto alt tag on my images, but any advice on how to get them indexed better is appreciated

It was only thumbnails and it was removed somewhere around 4.2.x. Latest robots.txt:

User-agent: *
Disallow: /app/
Disallow: /store_closed.html