A: "Why is a built in, golf ball shaped filter a good thing to look for when selecting a teapot?"
The golf ball shaped filter has more holes than a flat built in filter. The 3D filter also makes is very unlikely that all the holes get clogged with expanding tea leaves.
Then, there is the fact that it's made with the same material as the teapot instead of metal, which should not come in contact with tea. So, it filters without changing the taste of the tea.
A filter is most useful when you are brewing broken or very small tea leaves or if the teapot is very big. But, if you're brewing big, whole Wuyi Oolong or Wenshan Baozhong leaves in a very small teapot, then you probably don't need such a filter. Big leaves don't fit round holes exactly. And should one enter it, then you would use a bamboo pick to clear it open.
Golf ball shaped filters are rather recent inventions (40 years approximately). So, you would look for one in a new pot, but you definitely wouldn't want to see such a filter in an 'antique' one!
(Antique pot without filter)