These co-features were often filler, making it easier to pad the comic with the normal amount of pages and were a great way for publishers to try out new artists and writers. They were sometimes used the way syndicated television will use a less known TV show before or after one of their big hits. This was a great way to introduce new characters and test out creators as well.
The co-feature was widely used in Golden Age comics as well as some Silver Age ones. Most current comic books do not use the co-feature, but it has started to be used again in certain weekly comic books that Marvel and DC has been using to give the weekly artist and writer a bit of a break from having to write or draw so much in a week. They also started to be used in some of DC top titles, but were cut back to help lower prices.
It is also interesting to note that some of the characters from these co-features gained their own comic book series as they became as or more popular than the characters from the main title. Captain America used to co-star in the comic Tales of Suspense, but when his popularity began to rise, Marvel launched his own comic book that started out at issue #100 as his appearances in Tales of Suspense ended in issue #99, which was also the last issue of that series.