They are used to lubricate the bearing with sea water
where natural flow isn't enough. Usually some of the cooling salt water
is returned thru the stern bearings. My mains are setup this way. Most of the returning salt water
goes out the exhaust
, a copper 3/8" line goes to each bearing, and the remainder goes out near the engines.
Sometimes you see a pickup forward of the bearing housing forcing water thru by forward movement.
The grease helps lubricate the seal. Water pressure forces the grease against the seal where it contacts the shaft, increasing lubrication without much (if any) water coming thru the seal and greatly reduces shaft wear. It needs to be waterproof grease.