By default, photos with searchhidden do not appear in the search
results. This allows a small number of representative images from
a large album to appear in the results, while the rest can be
found on the album's page.
The same effect could be achieved with a tag and forbid search,
but tag searching has much higher cost and it would be more difficult
to implement as a default behavior without requiring lots of special
checks whenever listing tags etc.
voussoir