I'm not sure if I really understand this question, Oak and Sycamore both produce timber but for differant purposes, oak for example is suitable for furniture and beams for building construction or restoration work. Sycamore on the other hand is suitable for kitchen implements for its low odour but is not so durable outside.
So really it all boils down to demand. Hope that helps Tbird+
As a more general answer, all trees produce timber.
Because they just do.
It's probably because the word timber came into use to describe the solid parts of trees, excluding the outside bits, which came to be known as bark. Normally dead trees, but can at a push be used for live also.
Not all species of tree grow at the same rate. You may also want to take that into account before going into commercial production. You certainly shouldn't be looking for a quick return on your investment.