Multi-angle valve jobs are the most common way to make a valve more aerodynamic and make the head more efficient. A multi-angle valve job puts extra cuts on the valve near where it mates with the cylinder head, streamlining it and helping to move air past its surfaces with the minimum resistance and turbulence possible. OEM valve jobs are usually one or two-angle valve jobs. This is done to keep costs low, and even if they want more airflow, OEMs will often just switch to a larger valve instead of using multi-angle valve jobs because it's cheaper than performing the extra machining of a multi-angle job. However, for the average enthusiast, the opposite typically holds true because oversized valves can be very costly.
The more angles a valve job employs, the better it will generally flow. The most common multi-angle valve jobs are 3-angle, 5-angle and radius valve jobs. A radius valve job is basically a 5-angle job where the transitions between the 5 angles are smoothed out. The downside is that the more cuts, the higher the cost. For this reason, 3-angle jobs are the most common and offer a good balance between cost and performance. Don't underestimate the importance of a quality valve job.