Do you think that it's important to study consumer behavior before starting marketing campaign ?

Yes, it is very important. Infact they are interconnected with each other.  Understanding how to utilize the information from consumer behavior is the key that a company or marketers can focus on producing products and/or services to its targeted markets. The marketers also can figure out how to develop effective campaigns using their market know how.

yes

sure cause one of the major elements of the marketing plan which you will gide you in the campaign.

the reason of your marketing campaign is to target the customer and send them your message. how you can make sue they will receive the message if you didn't study thier behavior.

Definitely, although there is no need of relying on it completely. Studying Consumer Behavior will  aid in observing changes market trends.