Well, yes, partly it's to rule out MR. (Though someone in the mild range would be quite capable of scheduling an assessment, if it came to that.) It makes a difference in your specific diagnosis.
The more important reason is to try to spot weaknesses in your cognitive profile. For example, if you tend to score at a certain percentile for most things, and then randomly dip fifty points lower for math, it's a pretty good guess you've got problems doing math--maybe a learning disability in that area. Or, maybe you end up with a huge verbal>performance gap; that's a dead giveaway that they should look at something like NVLD even if your overall IQ is just fine. With me, for example, they saw that I was dead-on horrible at arranging pictures into stories when they had social content, which was a red flag for AS.
It's not really the overall IQ that's all that important; not with an autistic cognitive style. It's just that we tend to have a lot of strong points and weak points and not all that much in the middle--scattered subscores versus the typical NT clustered scores. You want to request your full report, not just the number but how you did on the specific tasks, and ask what that means about your strengths and weaknesses.