On boat trailers, bearing buddies or bearing protectors are bearing caps that allow the user to apply grease to the bearing cavity without disassembling the wheel hub. They allow easy greasing without a lot of work, time or mess.
The downside (in life there's almost always a downside), is that most users are tempted to over grease their bearings. When you over grease your bearings, you essentially cause the seal on the rear of the wheel hub to blow out. It's easy to see when this happens because grease will spew from the inside of the hub all over the inside of the rim. Now the seal no longer keeps water out and as you may have guessed, water and bearings don't mix well. In fact, the bearings almost always prematurely fail and this failure never happens at a trailer service shop... It's always happens at the wrong place and time.
Which leads me to my next tip... All trailer towers should carry a complete bearing set pre-installed and greased in the correct size hub as well as a spare tire. This way you always have all the parts to make a speedy repair roadside. Happy trailering!