You do need to repack your bearings if you have Bearing Buddies.
Bearing Buddies do get grease to the back bearing.
Although the bearing buddies not fully eliminate the need for proper wheel bearing greasing and hub and bearing maintenance.
And while bearing buddies push grease into the back wheel bearing, it's still required to manually repack the bearings and hubs with grease and replace the seals periodically as well.
Bearing Buddies are worth it, especially if you have a boat trailer which frequently gets submerged in water.
Bearing buddies help to prevent water from entering the hubs and and damaging the trailers wheel bearings.
Bearing buddies also are a convenient way of greasing your trailers wheel bearings and prevent water from getting drawn into the wheel bearing hub when the boat trailer is submerged.
However for other trailers that don't get submerged in water, the bearing buddies are not always worth it.
As long as you properly grease the wheel bearings when installing them on your trailer they should last for 10,000 to 20,000 or more miles.
When greasing wheel bearings you can do so by hand, but it's easier and better to use a wheel bearing greasing tool that you place the wheel bearings in and use a grease gun to force grease into the wheel bearings.
The wheel bearing grease tool ensures that you get enough grease into the wheel bearing to prevent the wheel bearing from going bad or overheating.
I use bearing buddies on my boat trailer but not on my other trailers.
As long as the wheel bearings are not getting hot or starting to make grinding noses or starting to seize up the trailer wheel bearings should last many thousands of miles without needing additional grease.
The bearing buddies not always ensure proper greasing of the wheel bearings and basically just pushes grease into the hubs which is a good thing but you don't want to add too much grease which could damage the wheel bearing hub seals.