Why do it?
Because work experience gives you a chance to develop key skills that are vital for the workplace, any training scheme and even further education. Being able to show that you have good communication, problem solving, teamwork and I.T skills could be the difference between a successful and an unsuccessful interview. If your dream is to work in a sky-scraper in the city it is likely you will need a certain level of 'know-how' in the office.
Although it may sound simple, just having basic office skills such as knowing how to use a fax machine and photocopier can be a bonus. Employers are very busy people and may not have time to show new recruits the basics, so if you can prove that you are already comfortable and confident in an office environment then you are already on your way to securing that job. It may also get you out of making tea for the first fortnight!
Work experience can also provide a good look into a certain career. A job that has always appealed to you could turn out to be completely boring. But you have to try it to know it isn't for you. On the other hand work experience could confirm your goals and get you on the first step of the ladder.
A common opinion amongst young people is that work experience is a waste of time, especially when you could be doing paid work instead. Although it can't be guaranteed that it will be a success, any experience on your CV looks better than none.
Part-time work is also a great way to build up your CV and increase your skills.