I am an undergraduate senior. I wonder why people keep doing page breaking in their thesis, like one diagram for one page. Is that for the page count? or just because they have to? Also, I wonder if it is a good thing to do in my thesis.
A reason for some is because LaTeX will do this. If you have Floating figures, and the layout engine can not work out a good way to place them, as to balancing the amount of whitespace, and other rules for an "good looking document" (eg No Figures on First Page is a rule for IEEE transactions) it will shunt them to the end.
The rules LaTeX uses to define what is a "good looking document" can (and often should) be adjusted. See for example this, where the rules for the various fractions of text etc to figure are adjusted.
Positioning can also be forced some, eg with
A lot of detail into how LaTeX placed figures can be found in this TeX.SE question.
Obviously this only applist if the thesis was prepared in LaTeX (or some other system with automated rule based layout, that prefers figure only pages to squeezing a figure in to a page badly).
In other cases, it could be because the page was inserted from the output of another program. Eg a simulator output printed to PDF, merged in. Or a scanned hand drawn diagram inserted as a full page image. This is not a good or attractive why to do things, but it is fast/convenient.
In general, as a undergraduate thesis is not heavily judged on its appearance, its often not worth the time to do better. I know when I was writing mine, I had 3 final year (and thus difficult/timeconsuming) coursework/project based units to do at the same time. I had better things to do, like ensure the wording was clear, and I didn't exceed the word-count. Placing figures well is hard, I was satisfied once I got FloatBarriers working, to stop the figures from floating all the way to the end -- FloatBarriers can be used to force figures not to be shunted beyond the current section (etc).
The undergrad thesis will likely be the first serious academic writing of that form they have done, and writing it comes at the end of the hardest work they have done, during or just before their final exams. Don't hold any up as perfection in method to judge yours by.
There is a quote (I believe from someone on this site) which basically said: "Your PhD thesis will be the worst academic writing you have ever done; because anything written after will be better as you will now be more experienced." This goes double for an undergrad thesis.
They do it because they do not know how to do it differently.
In general, pages shall be "full" -- either there is text on them with a figure on top or bottom, or the figures are placed on a separate page if the figures are too large or you get a lot of them. Of course, at the end of each chapter, you will have an empty space, between "nothing" and "almost two pages", because each chapter should start on an odd page (on the right when you open the book).
To make long story short: You should not have a lot of white space on pages around figures; the figures should be arranged in such a way that you get rid of the white space.