How helpful is a Fidget Spinner for ADHD or Autism?
Here are a few ways a fidget spinner for autism and other fidget toys can benefit your child:

Fidgets are often recommended by occupational therapists to provide the right amount of sensory input.
They can help children with ADHD or autism concentrate by helping them to filter out extra sensory information that would otherwise be distracting.  
Some fidget toys have smooth or soft surfaces to promote calm and eliminate anxiety.
Unlike fidget spinners that can be visually distracting, many fidget objects, like putty, are designed so children (and their classmates) will keep their eyes on the teacher.
Fidget tools can provide relief in stressful or high-pressure situations.
Fidgets can be suitable for all age groups — even adults.
Of course, even though fidgets are important tools, they can also be fun.