Sidekicks are a common thing in video games. They can serve multiple purposes. They can make for a better story, be used as an element that opens up co-op play, help solve puzzles or in most cases all of these at the same time. The relationship between the hero and his or her sidekick is very important and can determine how enjoyable the game is. Sam and Max is a good example of this, adding humor to the story. The sidekick also helps the player in his journey, giving him tips or helping him solve puzzles.
The presence of a side kick can also enrich the gameplay by opening up the possibility for co-op play with a second player. On the other hand, sidekicks can be very annoying when their AI isn't so smart, wasting ammo, dying and getting in your way.