I've done the IOWAs or the Stanfords every year with my homeschooled kids. (They just finished 7th and 5th grades, and have been doing them since K.) I do it partly so that they'll be VERY comfortable taking standardized tests (and they are), and partly to keep a GENERAL idea of where they stand. I say general, because I can see from year to year that they're improving. You can't necessarily trust the score for a specific subject for a specific year.
For example, last year my then-6th-grade DS, who is a HORRIFIC speller, scored "Post High School" in spelling, while my then-4th-grade DD who is a decent speller scored slightly lower than average for her grade level. But *I* know how well they're really doing, so I know to take those scores with a grain of salt. Now if they CONSISTENTLY scored low in a particular subject, I would realize that there might be a problem and put some extra emphasis on that in the future. My DS scored relatively low in math computation for a couple of years, and I realized that it's because that on that particular part of the test there are more problems than you can finish in the time allotted unless you work very quickly. He just didn't have his math facts down well enough to do it fast enough. We worked on that, and his scores improved in following years.
The bottom line is that standardized test scores may be useful for showing you general trends (yes, he is improving in his reading level from year to year...) or for pointing out weak areas. But don't get too upset if one section for one subject is low. Or even if everything is low, for that matter. He may not have been feeling well, or he may have been stressed (I test one kid every year who literally SHAKES on the first part of the math test), or he may have misunderstood instructions, or...
Edited to add: If you're homeschooling, they can also be useful to show to skeptical grandparents to "prove" that you are educating your children successfully!

(Not that academic achievement is all that there is to successful education, but at least you have something objective to show.)