Simply put, the aptly named button "More…" loads more results.
By default† the "All My Files" smart folder is arranged by kind and sorted by last access time. Arranging means those blocks that contains files belonging to its category type (e.g. Movies or Music) and sorting means the order the files are sorted within the block.
All My Files is all about you, so ask yourself: what would be your most typical use–case scenario involving this smart folder?
I use last access time for both arranging and sorting the folder. I have categories of "Today", "Yesterday", "Last 7 days", "Last 30 days" and "Earlier" and within every category the items are sorted by the last access time during its period.
Now, this is where the More… button shines: All My Files folder is a smart folder which means the contents are generated by a Spotlight search. That means the more results it tries to query the longer it takes to make the contents usable. Question is, would you rather
- see the most relevant files quickly
- see the whole lot of your files after staring at slowly updating, laggy, maybe blank Finder window for quite some time?
For reference, when both arranging and sorting the All My Files by last access time, the results I get after consequent presses of More… button are:
- Today: 2 → 3 → 4
- Yesterday: 14 → 15 → 16
- Last 7 days: 185 → 193 → 198
- Last 30 days: 354 → 364 → 373
- Earlier: 9445 → 19425 → 72145
After the second press the button is gone, probably indicating all the available files have been found.
Now the odds are the file I'm looking for can be found from the initial search (which already returns 10000‡ results)—and if not, the All My Files folder may not be the best tool to search for the file in the first place.
Honestly, at times I'd wish for a Less… button, for even more responsive All My Files usage; 10000 results are more than enough.
Finally, if you happen to realize the All My Files folder doesn't quite suite your workflow, you can, of course, remove it from your sidebar.
†) I'm not 100% if these are the default settings, but I could assume so.
‡) You might notice that after the second press I get a total of 20000 results—there's some logic applied!