helm-projectile makes you set a project before trying to find files or directories, or searching through files with ag. All the searches are then based from the project root.
How does one start, for example, a helm-projectile-ag search from within a subdirectory of the project?
Documentation is quite sparse for helm and helm-projectile so I'll leave this here for posterity.
One way of searching through files in a particular directory would be:
C-c p d find the directory of your choice, then
C-s allows you to specify a pattern to search for within that directory.
Something similar can be done by searching for files with C-c p f, highlighting some with C-space and then using C-s to search through the selected files.
This link provides more examples but isn't always easy to follow.