It's not a black and white "either you are an illusion or you are not" mentality. I'm not saying that everything is an illusion, I'm merely stating that it is a possibility- a possibility that is essential to acknowledge. It's not a conclusion.
Our entire lives, everything, is based on this reality. Although there is evidence that we exist, the manner of existence (reality) is an uncertainty.
This uncertainty ranges from world views (think Medieval world view versus todays world view) to the idea that perhaps we may be "plugged into the matrix" without a clue. As physical reality is the basis of much thought (almost all thought in science) then the importance of such an acknowledgment is rooted in the fact that the basis of this knowledge may be an illusion (or delusion). There is no strong evidence leading either way. This is why a level of objectivity as well as subjectivity is beneficial to a thinker.
Here's an example that might clear things up:
Belief in a higher being, for example, like philosophy, is not only rooted in our inquisitive nature, but also rooted in our intense level of metaphysical uncertainty. God or gods may exist, spirits, demons, angels, faeries, and trolls may or may not exist. This uncertainty is the basis of such belief.
(as for wording: although a single word in its own may be nebulous, context provides words with a level of precision. for example, in the English language, articles, prefixes, suffixes, adjectives, etc make words more precise. I'm not sure how precise my wording has been thus far, but for the most part, i've been using wording that i have found common enough in the philosophical community that i didn't feel the need to clear them up. this is because i often don't have enough time on my hands to make such lengthy posts.)