This question has been asked and debated many many times in numerous threads!
There seems to be a number of reasons why adding weight works, here's just a few:
1) Until recently, most buggies were designed in the pre-Lipo days. Nimh's were heavier so the easiest way for people to get up to speed with Lipo was to add weight so the overall weight was about the same. When doing this, you could also alter the weight bias. This is similar in principle to how people used to move batteries backward or forwards on the chassis.
2) The cars are so much more powerful now than in the XX-era; a heavier car tends to accelerate smoother and handle the bumpy stuff better.
3) At regional and national level, practice time is usually very limited so it is easier/quicker to add/remove weight to dial the car in than try and fine tune the suspension in that limited practice time.
Adding weight just works! Spark up a conversation with Tony "Rudebits" Evdoka if you need convincing further
