What is the point of tension in Dragon Quest IX/other DQ games

The higher your tension, the more damage you'll do. Tension can be at 0, 5, 20, 50 or 100. The higher it is the more damage you'll do, the more likely you are to hit, and critical* hit. At 100 you do MUCH more damage (about 2x I think)

*not positive about the critical.

For me, with a normal uber falcon blade strike I do ~250 dmg. With 100 tension I do 1999 dmg (regular max. Dmg) If you attack with tension, it has increased power and your tension goes back down to 0. Tension can be gained using Egg On, Phsyche Up, Tension Boost (Gladiator Coupe de Grace) and Feel the Burn Stance. Only the hero can use Egg On. Phsyche Up is learnt by the Martial Artist skill tree, Focus. Feel the Burn Stance is learnt by the Gladiator skill tree, Guts. The effects of Tension cannot go beyond 100, and can be removed by enemy using Disuptive wave or lowered using Morale Masher. Tension is only a temporary buff; it is not carried over into the next battle. Tension cannot raise damage over the regular max. Dmg limit, with a few exceptions. Using Oomph, 100 Tension, Double Up and Double-Edged Slash will kill you, and most probably the enemy. You are more intimidating to an enemy with more Tension, increasing their chances of fleeing the battle, making it a bad tactic for fighting metal slimes. Enemy can also raise their tension using all the skills listed above, with the exception of Tension Boost, as enemy are unable to use Coupe de Graces.

if i remember correctly Tension in DQ9 maxes out at 100. If a character has maximum tension they will deal significantly more damage on their next attack (melee for sure, not certain on magic).