For Saturday Night Live, the host is a key part of the show. With no host, there is no show. The host for a show is usually chosen based on popular culture and who is popular at the time. The host of the show works with the cast the whole week before the show to work on their parts and work out all of the kinks so they can perform on the same level that the cast of Saturday Night Live does. All cast members are professionally trained in improvision so the host needs as much practice as possible before the show.
Over the years, a variety of different people have hosted Saturday Night Live. This ranges from Justin Bieber to John McCain, from Betty White to 7 year-old Drew Barrymore. Many Presidents and Vice Presidents have made and apperence on Saturday Night Live in hopes that they can reach the comedic majority. Saturday Night Live likes to claim they have influenced Presidential elections one way or another, which could be a possibility.
Like with anything, there will always be people that are great at something, others that are decent and those that should find something else to do. This also applies to hosting Saturday Night Live. Some people have hosted Saturday Nighht Live multiple times while others have barely made it through one show.