Sun Yat-Sen is the founder of the Republic of China (when China ceased to be an empire and the emperor was demoted). When this happened, China took the name of the Republic of China (ROC) and adopted the red and blue flag with the sun that Taiwan now holds. But when the Communists arrived with Mao Zedong, they were renamed the People's Republic of China and changed their flag to the Communist one. Taiwan, headed by Chiang Kai Sek, remained the last stronghold of what was once the ROC, and that is why they consider Sun Yat Sen the founder of the nation.
The Memorial itself is not very interesting, but it is one of the most photogenic spots in the city to take photos of Taipei 101. The best spots are the south entrance of the memorial, and the entire lake area of Zhongshan Park (which also enchants children because it has turtles and fish).
When you finish walking through the gardens, you can go to dinner at the famous Aunt Su's Pizza House, whose specialty is pizzas and fried chicken. You'll probably have to wait a while to get a table, but they tend to be quick and the night views of Taipei 101 from there are impressive.
