One reason is that sometimes companies claim money from the government for their R&D spend (some sort of scheme by government to incent R&D spending, presumably). In that event the government wants to prevent companies from gaming the system by simply saying that they spent all their profits each year on R&D. So rules are created that say you have to spread the spend over N years for tax purposes. Same as asset depreciation -- you're generally not allowed to depreciate a large asset in one year.