i have 50% DR and 75% Block
what the hell is defense for
does it decrease chance of being hit before the block
or just a useless number

Less change of being hit
If you are hit. Then Block chance comes into play
If you are hit instead of blocked, them DR% comes into play

Think it worked like this

most attacks and attack-based ability require a hit check. the chance of hitting an opponent is:
chance to hit = 2 * alvl/(alvl+dlvl) * ar/(ar+def)
this means def lowers the chance to get hit, but with diminishing return
if the hit check is successful and the attack is blockable, your block chance comes into play. if the attack is not blocked, then dodge/avoid/evade are considered. if the attack is not blocked and not dodged/avoided/evaded, the physical dmg is reduced by dr