Some people view the taxation of inheritances as a limit on inequality (see part 1), however I think a more key reason is that it prevents a family from accruing massive ownership which would eventually change the power dynamics for government.
Witness Lord Ashcroft's woes in Belize, where he controls a great deal of capital relative to the total size of the economy. The government's taking of some of his property works like a "life tax" to limit his power there. 100 years ago, this open warfare would probably have resulted in redcoats making a cursory visit to decapitate Belize's president, or were it not a British Lord, perhaps we would have seen an old man in jail on trumped up charges. Today (thankfully), we see a less deadly power struggle between an individual and a government.
Self-preserving governments should act to avoid such struggles, so they tax the accumulation of wealth.