You should search more... someone's probably going to close this as a useless thread.
But as an answer, tuner village is a site where you can buy tuner cars... that's the primary reason, but also, if your car is a popular make, or is a japanese car with performance potential, you can buy the stage 5 turbo for it and/or the original suspension.
Stage 5 Turbo gives you almost exactly the same power as Stage 4 (sometimes more, sometimes less, I think), but has a more linear power curve, thus, makes your car faster even IF it doesn't give as much hp.
The Original Suspension has more travel rate, more height settings, bigger camber values and a simpler damper setting system than a full-racing suspension.