Imprints can be small or large initiatives, and like any comic book can go the way of the dodo and be cancelled as in the case of the DC imprint Minx, which was a manga geared towards girls. Some imprints can continue to grow and thrive with the right titles.
As stated before, one example of why a publisher would want to create an imprint is if they want to produce kids comic books, but whose usual line is for the more mature readers. BOOM! did this with their Kaboom! imprint when they nabbed the rights to the Peanuts and Adventure Time comic books, among others.
Sometimes the reason is the opposite, such as in Marvel's case when they started the MAX imprint to release comic books that were geared towards a more mature audience. This covers the publisher and allows them to distance their main line of comic books away from any derision that could come their way from fans that don't approve of that kind of content. Marvel also has an imprint for creator owned content with their Icon line. It may seem odd that Marvel has a creator owned line, but this allows Marvel some wiggle room for some of their more high profile creators as many of them have exclusive contracts to work with Marvel and may not publish with anyone else.