It all depends on what you do during the gap year...
Sure, it's all well and good travelling around - but gap years would be utterly useless if you spend all your time laying on a beach doing nothing all day.
However, if you're travelling for a purpose such as volunteering or fundraising for charities, NGO's and the like, then yes - gap years are extremely useful - it shows that you've got an active interest in getting involved and supporting the wider community!
Also, gap year internships and programmes with firms could also boost your work experience - that also helps with university admissions, in addition to demonstrating to future employers that you've got considerable experience to bring to the table.
So yeah. Gap years are extremely useful if you spend it productively but travelling around, having a slumber in hotels and playing volleyball with bikini babes certainly DOESN'T build your character.