The Beginning of Tao

I just began to read Tao Te Ching, and I'm beginning to tag it as one of the strangest books I've ever read. I don't mean it in a bad way, because I'm actually starting to like it a little. What strikes me as weird of the text is that it bases its teachings upon very common aspects of life we can relate to, but we have never thought of before in that way. He uses a very common thing like a door to show us how important empty space is.
For example, "Cut doors and windows for a room; it is the holes which make it useful. Therefore, benefit comes from what is there; usefulness from what is not there." (pg. 13, chapter 11).
I had never thought the real usefulness of a door was the space in the middle which lets you in and out. If you think about it, the main difference between a door or window to a wall, is that it has space, which make your house accessible to light, wind and people.
However, I wonder what the difference between usefulness and benefit are. Wouldn't the usefulness of something be beneficial for you? In some way or another, something useful will be useful because it benefits you. So maybe, the useful and the beneficial are somehow connected.
 Also, if you look at it in another way, there are many examples in which I would doubt a useful thing is not there. For example, a lamp is very useful, and there's nothing taken off which makes it useful. The space around the bulb doesn't give the light, the filament with electricity does. In fact, many new light devices have eliminated that useless space to make smaller and more useful light devices.
In any case, I hope to find some explanation ahead in the book to explain my doubt over useful vs. beneficial.
The rest of the teachings I've read until now are very interesting too, and as I read, I hope I find some magic revelation that will help me understand them fully. I'm sure there's some important concept that connects all the teachings, and I'm curious to see if I'm right.
I'm also wondering what the Tao really is.