I have never tested my kids until this year when they started taking gym and art class at the ps. I was glad to get them tested at first but they test 3 or 4 times throughout the year and that was toooooo much! However as far as the results went; as someone else mentioned, I didn’t learn anything that I didn’t already know. I knew my oldest DD was behind in math and it showed. But it gave me reassurance that I knew what needed to be done.
I’m not required to test in TX. Years ago, we did use CAT through a correspondence school. Nothing brought to light that I didn’t know. That said, I do discuss/demonstrate/practice test taking from time to time. It may be a timed math test, spelling or vocabulary quiz (rarely,) or a request for written narrations. By far, the most important skill imo would be the old style “Blue Book” written test. This is certainly similar to the written term examinations of which CM discussed. All the years of oral/written narrations are building toward this testing skill. If you’re doing this, you know what your children know (oral, written, or both.) Throw in some math tests from time to time and you’re way ahead of those of us who don’t formally test. But, then, you’d have to ask if it’s necessary in your situation in the first place. ;0)