How are they supposed to support the PLA navy if all they have is a boat with some military radios? The only thing I can think of is cannon fodder or search and rescue.

The OP gives an example:

In the South China Sea, the Spratly Islands have attracted most attention as the Chinese government has built artificial islands on reefs and shoals in these waters, militarizing them with aircraft strips, harbours and radar facilities. Chinese fishing boats bolster the effort by swarming the zone, intimidating potential competitors, as they did in 2018, suddenly dispatching more than 90 fishing ships to drop anchor within several miles of Philippines-held Thitu Island, immediately after Manila began modest upgrades on the island's infrastructure.

Other obvious uses are espionage, observation, and covert transportation.