Scoring a perfect century in test match especially on an international level is something that very few batsmen manage to do. While it may be regular for batsmen like Virat Kohli and Steve Smith to do such things, Rahane and Williamson as Captains of their respective team decided not to fall back. They both scored a century on the second day of the test match that New Zealand was playing against Pakistan and India were playing against Australia.
During the 2nd test match that was happening between New Zealand and Pakistan, Kane Williamson held the fort for his team and scored not just a century, but a double century for his team. He scored 238 runs in 364 balls that he faced and his performance can only be sensed from his stats of this innings. New Zealand never had to bat in the second innings since the pace attack led by Tim Southee and young Kyle Jamieson bowled out Pakistani batsmen at only 186 runs in their second innings.
Rahane, on the other hand, might not have scored a double century like Williamson, but his century helped India gain a win against Australia in the second test match. He made 112 runs in 223 balls which gave India a solid lead and allowed them to acquire a win. So both the tons that both these players scored are very special and deserving in their own way.