I didn't watch the Jenner interview. However, I believe it serves a very useful societal purpose for someone who is already well-known to come out as transgender. In just the past 5 years, there has been a major swing in public sentiment regarding same-sex marriage. That change in public attitude didn't happen by itself. It came about because over the past few decades, more and more people came out as gay. Gay characters began to appear in movies and on TV. More and more people including celebrities, politicians, athletes, family members, friends, co-workers, and neighbors came out. All of this began to snowball, and suddenly most people knew someone who is gay. It's hard to hate someone or discriminate against someone who you already knew and liked before finding out that they happen to be gay.
Jenner going public is an important step for the society's understanding and awareness of transgender individuals. It is important for adults, but it is also important for kids to know that transgender people exist. Some of those kids, and some the parents of those kids, will eventually face the issue personally.
Kardashians, OTOH, serve no useful purpose that I can see.