A web site's navigation menus usually get placed at:
a) the top area of a web page - filling the header, that is;
b) the top (the header), on demand - by means of a hamburger menu button;
c) top + left-hand column;
d) top + footer (the footer actually acting as mini site map).
Navigation must be immediately visible - so as to let the user know where they are in the whole structure of the web site and it must be easily available when the user would want to go somewhere else. With these two requirements on mind and given that the user's gaze usually starts from the top, it makes sense to place navigation in that area.
BUT: on the other hand, imagine the scenario of sending a link to a friend to recommend them a particular service or product. Your friend receives the link, they are eager to learn more, but they get lost in the header, with its detailed menus, plus distracting hero images and the like...
Especially people with lower cognitive ceilings, elder people and people with shorter attention span, may just close the page, thinking what they were looking for is not there or simply forgetting what they were about to do in that page...
Now imagine how much more effective it would have been if there was a very succinct header, containing the logo, a breadcrumb and a button leading to full and detailed navigation... at the footer.
You immediately see the content you are interested in, no distractions, no half screen real estate taken by navigation and strong visual distractions (hero images, etc), at the same time you do have an idea where you are in the structure of the whole web site, you do have a link to the home page (the logo) and you do have access to all the other menus, on demand.
We have already seen this, as described in option d) above. (Actually, my personal knee-jerk reaction when needing to navigate around a web site is to scroll down to the footer, because that is the only area not overcrowded with visual distractions and with really detailed navigation menus. Simply text in columns and no fancy stuff whatsoever. "How boring and useless!", a graphic designer would say...) The difference between d) and my suggestion is that in d) the header still contains plenty of stuff, distracting you from the page-specific content.
"Don't we achieve the same with option b)?", you may ask. Well, in a dropdown menu you cannot go into the same degree of detail as you could in the footer. Given the unlimited space you have there, you could go as far as putting a full site-map.
Of course, this header approach does have its limitations when it comes to web sites where you have more things to interact with, rather than things to read - e.g. a booking platform. But wait - if interaction is the main thing to do on a web page - why not have all its buttons and menus inside the main content area and till keep the header succinct?
We are used to having the header come with strong visual weight, BUT shouldn't the main content section of a web page come first in the page's visual hierarchy, rather than the header?
Could you point out specific disadvantages to a web page layout of smallish header and detailed navigation (site map) in the footer?