Star Trek: Every Borg Ship Explained

So far we've only seen one Borg Probe onscreen, in the Voyager episode Dark Frontier.

The probe was sent by the Collective to assimilate the crew of Voyager, but Voyager was lucky to only face a Probe. Borg Probes are tiny compared to their other vessels, about half the size of Voyager, and with very comparable firepower. Voyager was able to defeat the Probe by lowering their shields and beaming a photon torpedo near their power matrix.

Not much is known about the exact purpose of Borg Probes, but the name "probe" suggests that these vessels are used to gather information, likely to seek out targets for assimilation. The Probe from Dark Frontier probably only attacked Voyager directly because Voyager was such a small ship and no other Borg ships were in range.

So, while their name and size suggests that these vehicles are typically only used for surveillance, we've seen that they are sometimes diverted to other tasks when needed.