Strictly speaking, interests are not totally necessary, there are no hard and fast rules on this, and you are not obliged to include interests when you write a CV.
Nevertheless, they are quite conventional, and most people include them if space allows. At the very least they tell the employer something else about you. This can be a good thing, or in certain cases it could be disadvantageous. For example, if your hobbies are dangerous sports, then it could actually deter the employer.
Particularly helpful is if you have interests which are relevant to the job you are targeting. Although, not everyone has interests which are relevant to the job.
Keep fit and sport - these show that you lead a healthy lifestyle, and some employers may take this as a sign that you may spend less time off sick.
Friends and socialising - whilst this doesn't necessarily seem work-related, if you are a people person and social skills are important for many jobs, and demonstrating this can help in your job applications.
In positions of responsibility are also useful to include sometimes, so for example if you chair board meetings, and particularly if it is something related to the job you are targeting, then this could be useful.
Things you may wish to exclude from your list of interests include potentially generous things like rock climbing, skydiving, parachuting, paragliding, scuba-diving, boxing, rally car driving etc.
You should use your common sense and decide which interests are worth including and which ones you can leave out.