slackpkg itself actually does just wget and install binary packages. You may be thinking of sbopkg or sbotools which grab SlackBuild scripts and source tarball

slackpkg itself actually does just wget and install binary packages. You may be thinking of sbopkg or sbotools which grab SlackBuild scripts and source tarballs from slackbuilds.org, builds and installs the resulting packages. They are extremely useful tools every slacker should have, but they're third-party (as is slackbuilds.org) and not officially a part of Slackware.