What's so interesting about them is that logos are ridiculously hard to do well. 

Why? They combine elements of visual communication, symbolism, typography,  design, and marketing communications. They must work well on huge billboards, web sites, packaging, business cards, t-shirts, coffee cups, and more. They must work in color, greyscale, and monochrome.  

It's hard to quantify why a good logo is so good. It must convey meaning and at the same time be simple. It must be visually appealing and support what the brand is trying to say. It must accomplish all this in a handful of shapes, lines and colors. For really big firms, a logo has to work globally. That means it can't offend foreign cultures or, often, it can't rely on the English language. And it has to work in a tiny little space.

Big companies will spend big money on them because a good logo is worth so much.  

Here's an interview with the person who created one of mankind's most recognizable and valuable logos - the logo of Apple Computer. I thought you may enjoy reading the real story: http://creativebits.org/interview/interview_rob_janoff_designer_apple_logo