Could someone tell me what's the use of observers when capturing a wild pet?

Catchers have two functions: they can see where a pet is coming from when it comes to their area, and they can potentially catch the pet. Observers can only do the first. This way, you can try to predict where the pet will go next cycle, and they're cheaper than hiring a catcher. If you already have the Cartographer's Guild built, or have a Clairvoyant in your Inn, the observer is mostly redundant. 

@Sumatra_Echo Thanks very much, that was precisely what I needed to know! :)