Why paginate/page break an infinitely scrolling dynamic list?

Question

There is no shortage of pagination questions on UX.SE.

Still, something struck me recently while looking through Google's Image search: it's a dynamically-loaded, infinitely-scrollable list with (some form of) pagination, which seems like a unique case.

I mulled over a few reasons for it, but discarded them:

1. As an anchor for content: "Hey, Joe, search 'pagination' and check out page 2". This doesn't work because Google tailors search results per user, so my page 2 won't be Joe's page 2.
2. 'Page breaks' demarcate loading zones. Not the case; content is dynamically loaded between Page breaks. Even then, why would the user care?
3. To show how much you've already loaded. That's why scrollbars have dynamically-sized handles.

Why would a designer implement this? Is there a genuine advantage to the user that I'm missing?

Example of the current behaviour:

Answer 1

I would say it's probably an artifact of the days before the infinite scroll. Probably there to keep from confusing older or less-tech-savvy users.

Possibly it's an engineering remnant, they already had everything broken up in some way and so it was easier to include the pagination than to remove it.

And a third possibility: The use case where a user comes, sees an image and wants to find it later, they remember they had to get to page 3 to see it.

Answer 2

Infinite scroll can have real performance problems and make for an irritating experience. http://www.fastcodesign.com/ is a site that (for me at least) is overly intrusive with its automatic loading it could use a pagination model to solve this.

Beyond the possible performance issues, there are many usability problems with infinite scroll. It can work but is often abused.

Answer 3

Performance is the most common reason for this. Loading content upfront is very costly for very long lists. Modern interfaces usually load dynamically on scroll. However, this can be limited as well because you do not want to blow up your user's browser. Hence you should be conscious of the performance cost of enlarging a page with more content. That being said, most modern equipment can handle quite a bit of abuse.

Another reason for this could be to give the user landmarks in his navigation. A more common example are books. You use pages for references when looking up information for yourself, e.g. "this was a dozen of pages ago". Without a way to break the larger information space in navigable chunks, it would be highly improbable that you could effectively jump ahead or back, e.g. "this was 200-300 lines ago".