This is complicated, but the designation could unlock more services for your child.
Many disabilities are VERY specific. Like a person might have trouble reading things that are written on a chalkboard, but they do fine if the information is written on paper.
I know a student who had trouble with handwriting. He was in middle school, and he still had trouble forming letters. The school was short on funding, so they only had a few computers, and this was the time-before everyone had a cellphone. One day, he was given a computer keyboard, and all of a sudden 5 pages of writing came pouring out in less than an hour.
I will never forget the look in his eyes when he turned around to say, "Typing is SO much easier for me!" He smiled for days. It was a major turnaround for his education, because typing enabled him to work at grade level. His life must have been so frustrating before that day.
Testing for disabilities doesn't catch every disability. A member of my family clearly has mild dyslexia, but always scored just out of range. They have long struggled with spelling, and other reversals, like swapping a plus for a minus sign in math. They have lifelong inability to discern thier right side from their left. PE class was a struggle. The teacher said go left, and they went right, while the whole class laughed at them. They came to hate PE and most sports.
In both cases, a small accommodation would have saved years of silent suffering.