If someone would watch a television show or even a film, at the very beginning, the company who made the film (e.g Producer) would have made an intro for their company. Some of the intro's represent the film. Idents are known as producing a companies brand. They let the audience know that when they watch a film, or television show, they'll know who made it. Idents are also used to keep up a good reputation.

For example, Sky Movies has made many idents for their many different platforms involving Sky Premiere all the way to Sky 90s, following all the different genres. The purpose for Sky movies using idents for certain types of films such as Star Wars and Disney, is to show the audience that Sky movies is the home for family entertainment. As Netflix, Amazon Instant Video and Now TV are all competing with each other. Mainly because you can watch unlimited amounts of films for a cheap price. Sky movies feels threatened by this so they enhance their brands by showing the audience that they're the home of entertainment.
The main purpose for dents is for companies like 20th Century Fox and Sky movies etc so show off their brand. idents are brought up in the beginning of the sequence so the viewers will get an understanding of who produced the sequence, if the film was a big blockbuster that everyone all knows and love, they change the ident to replicant the film so the fans of the film can get excited as it brings out the suspense in the film.
All different idents achieve different things by how they bring out their company in one short sequence. For example, as I spoke about earlier when they make the ident relate to the film, doing this will achieve an excitement towards the film from the audience. While when they show their original ident, this shows the viewer who produced the production. Many different idents have a different particular style they use when broadcasting their ident before the film or television show. For example, many of them just like 20th Century Fox just states their name without anything else behind it, whereas DreamWorks connects the imagery with the title because the sky is a dream like state.