The recent movie The Adventures of Tintin: The Secret of the Unicorn is a performance capture animated movie. What is a "performance capture" movie and what is the need for this technology when the same effects could be visualized using normal animation (think movies like Ice Age)?
Performance capture generally refers to the practice of capturing very subtle movements from real actors and using those in animation. Movies like Ice Age are fine without performance capture because the characters aren't realistic.
The problem you run into when trying to make realistic human animations is that the human brain is wired to detect very subtle cues in facial patterns and if an animator makes even minor errors, we tend to be uncomfortable with the animation, associating the problem with a physical defect, illness or death. That phenomenon is usually called the uncanny valley.
One thing you can theoretically do with this performance capture technology is record a broad array of facial expressions with a famous actor, and continue featuring that performance in new movies long after their ultimate demise. That seems even a little creepier to me than bad animation, but to each his own I guess.
Ultimately though, the reason for the technology is to capture an actor's well honed "art" as opposed to something an animator can realistically replicate.
I think performance capture is used to make animation that is more naturalistic through capturing movement from actors. This has been used with great effect where animation has to interact with live action - such as Gollum in the LoTR movies or King Kong (both using Andy Serkis as the actor), where a 'simply' animated character can be unconvincing. In this case it is an attempt to get around the 'uncanny valley' problem.
In a fully animated movie - I guess it is just an attempt to create a different look and feel, alongside tradional hand-drawn animation or CGI, it is neither better or preferred - just different.