Why should I buy textures if I can get them for free elsewhere on the web?

True, you can find a lot of sites which offer thousands of textures for free download. However, this so-called textures are mostly just simple photographs of surfaces or objects; often available only single-layered, non-seamless and in low resolution. Sometimes you may get some additional maps, such as bump or specular maps. But these maps often turn out to be just plain gray-scaled versions of the texture map with adjustments of brightness and contrast. That's usually what you can get for free. You will have to decide whether or not that is enough for your purposes.