I was diagnosed as an adult with both, so not sure how relevant it'll be, but I'll answer anyway.
I got the ASD diagnosis first, about 7 years ago, and it was helpful for me in finding an appropriate therapist who knew how to work with me on my mental health problems, helping the NHS mental health services understand my difficulties, and getting useful accommodations for my college and university courses.
I have a lot of difficulties with my executive functioning, which were identified during my ASD assessment and described as part and parcel of my ASD. Lots of ASD assessors will prefer to fold things like executive functioning difficulties, sensory difficulties, and coordination difficulties into the ASD diagnosis, rather than separately diagnosing ASD and ADHD, SPD and/or DCD/dyspraxia, and to be honest that approach makes sense to me, as these issues so often occur in autism and it's hard to separate out what's what in any one individual.
While the accommodations I had were useful to me while studying, I still had difficulties which weren't entirely addressed by accommodations and study techniques and such.
After I finished my degree, with a lower classification than my tutors thought I was capable of, I looked into ways of managing these difficulties.
I found research that suggested that for people with ASD who have symptoms that are also seen in ADHD, and which in ADHD are treated successfully with medication, ADHD meds can be helpful. But in the UK, these meds can only be prescribed if you have an ADHD diagnosis, not if you only have an ASD diagnosis with ASD symptoms that also occur in ADHD. So I went for an ADHD assessment earlier this year, to see if my neurodevelopmental differences would also fulfill ADHD criteria, and they do.
In other words, the ADHD diagnosis is necessary if you want to try certain medications that can be helpful for a subset of people with ASD, and I wish I'd been able to try them before doing my degree.
Otherwise, I'm not sure what the additional benefits would be, as many of the accommodations and techniques are the same whether those symptoms come from ASD or ADHD. But the situation may be different for your daughter, particularly as she's young and in education.