In our modern economic system wealth accrues more wealth. If you don’t actively redistribute the wealth, not just the income, then you end up with wildly unequal societies, in essence what we have today. Wildly unequal not just in terms of resources but also in opportunity. If you believe that equal opportunity should be a birthright of everyone then a sensible way to deliver a more equal opportunity for each new generation is to redistribute the wealth tied up in inherited estates from those with a lot of opportunity (the inheritors) to those lacking opportunity.
Do you understand it better when put like that?