The idea of hierarchy is a major part of design. It can be 'invisible', just like good typography, which is why inexperienced designers don't always have a good eye for it. Whether its achieved with the layout, the type size, colours or other means, hierarchy always plays a role in design.
I stick to the motto 'if you try to make everything stand out, nothing will stand out' because I've come to realise that people often confuse two separate issues: making text big enough to read, and making text big enough to be seen. Putting a telephone number on an advert, for instance, is not going to make people pick up the phone – it's purely information. It needs to be large enough to be read – and no larger. Whereas, if you had a unique selling proposition to shout about, it makes sense to bump up the type size.
I find that, especially when you're designing an advert for print, you should be able to first make a list of all of the text information in order of most- to least important. The text that's written to grab attention will be at the top: the text that is solely information, will go at the bottom.
The advert below shouts out 'vote yes' but it's unclear where to look next because of the confusion between the two columns – one is left aligned, the other centre aligned, and neither takes charge over the other because the contrast in type sizes is too subtle and the layout is inconsistent. It's an important message that was perhaps weakened by a design that was of a lesser standard that it should have been.
In this advert, right from the beginning the meaningful text struggles against the logo (because a poor decision was taken to add a tag line underneath it that requires the logo to be at least big enough for the tag line to be read). The designer was very reserved in their use of colour but there's a definite case of wanting-everything-to-stand-out-syndrome. The telephone number is some of the largest text on the design – but is it really going to sell holidays?
Ogilvy is the master of adverts that don't shy away from giving readers something to read. The headline is engaging, and the introduction gives you a reason to read the bulk of text that follows it. The visual hierarchy is admittedly simple, but there's no reason to ditch simplicity for 'creativity' when it comes to selling an idea or a product.