how much does sepuku help and how does it work? pls help

1.what does seppuku curently do? like how much does it buff? bc on the wiki

2. if i want the effects fo seppuku,do i have to use it on the weapon i want the effects to work on? like ive seen ppl use seppuku on a dagger and then have the effects on a nagakiba or rob.Bc im powerstancing rob and nagakiba with rob right hand for corps piler,so can i just use seppuku on the nagakiba or on a dagger and then use naga and rob to get the effets on both swords?