In my opinion you would need an active preamp if your system does not have enough gain. Today, most digital sources output in excess of 2V while most amplifiers reach full output near 1V. Suffice it to say that there is plenty of drive from the source to power the amp directly, so a simple attenuator in between the two like the pot in the box is all you need. There may be times due to impedance matching that an active buffer is preferable. While an active device it applies no gain. Nelson Pass actually wrote a very good paper on this topic that you should seek out.
Back in the vinyl days active preamps were more a necessity due to the gain required for phono cartridges, especially moving coil. At that time though full function preamps with built in phono stages were more the norm.