Why is it usually better to make a weld in the flat position

When a welder must certify for their appropriate welder's certifications, all of the samples are basically flat work. Simple tack welds to deep fill welds are required.

Explanation:

If you are welding two pieces of metal together, having the work as flat as possible allows for the best access for the weld to be proper. There are often more times then not that the work will not be in a flat position so if you are really just starting out, your practice welds should be made on flat work to get the skill necessary to weld well in other positions.