Plato
"Wise men speak because they have something to say; fools because they have to say something."

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The balance could be upset because the Webway was not invulnerable. For example, the Sixth Doctor once explained that the destruction of the planet Earth in 1985 would have disrupted the Webway. Lytton also spoke of the destruction of Mondas in 1986 as an event woven into the Webway. (TV: Attack of the Cybermen)
Mondas, as part of the Webway, was destined to be destroyed, and this could not be changed. If this event were rewritten, the Webway would be damaged, with catastrophic consequences for the entire universe. Billions would die, powerful civilizations would disappear, and the Doctor was not even sure that the fabric of reality of the universe would survive such a shake-up. (PROSE: Attack of the Cybermen)

-The Web of Time is not so much a fundamental aspect of the universe's existence as it is a meta-structure created and inserted into history by the Time Lords. It could not have existed until Rassilon constructed the Eye of Harmony.
-The Web is sometimes called the "chronology tether" (AUDIO: Neverland), and sometimes the "thread anchor" (PROSE: The Book of War). It is likely that it allowed Rassilon to choose which species would evolve, as he did with, say, some of the Time Lords' potential competitors. (AUDIO: Zagreus).
-Although not a fundamental aspect of the universe's existence, the Webway maintained a balance that reflected the holistic nature of time. (TV: The Two Doctors)