Transfer pricing is a really important issue especially nowadays that the economy is less restricted to boarders.
First of all, consider a group of companies. Imagine that the companies of this group manufacture products and sell both in the group and outside the group. What should be the amount reported as cost for the intra group transactions? It has a huge impact when it comes to financial reporting and to tax issues. If you consider that tax changes from country to country and each country is interested to maximize the tax revenue, you can understand that transfer pricing is an important issue.