Taiji Fist and Taiji Sword were naturally the pinnacle of Zhang Sanfeng's lifelong cultivation, absolute martial arts skills derived from the Taiji principle of yin and yang and the infinite variations. Zhang Sanfeng had defeated countless foes and secured Wudang's prominent position in the martial world with just Taiji Fist.
In the martial world, power is paramount. Strength brings the opportunity to talk about social niceties. To negotiate with a tiger, one must at least be a wolf; a sheep cannot hope to strike a deal with a tiger. Thus, Taiji became the foundation of Wudang's survival.
As for Pure Yang Wuji Skill, it encapsulates Zhang Sanfeng's Daoist cultivation essence, being Wudang Sect's most advanced internal skill, practiced only by Zhang Sanfeng and Yu Lianzhou, the sect leader. Even the senior disciple, Song Miaosong, was not privileged to learn it.