With autism often comes rigid behaviors. Kids get "locked in" or obsessed with one thing. We've had our fair share of obsessions. About three years ago, Luca was 5 and was obsessed with colors. He used to spend hours writing down the names of every color in the crayon box. Flash forward to now, he's almost 9 and we've come back to the obsession with colors.
Here's my 2 cents on the matter. If the the obsession is causing harm to the child or anyone else, it is something that needs to be shifted. If its harmless and just gets annoying, let it be, and go with them into the obsession. That obsession can then be used to create focus.
You can help your child focus if you use the obsession to teach them other things. We love the paint wall at Home Depot. We often come home with stacks of them. We'll use them to write his list of school tasks he needs to complete, or chores. Because it's on one of his 'color cards' it captures his attention and its easier to get him to do the things I need him to do. I have them around for visuals when we are learning math, or we make lists of adjectives, adverbs, etc. The point is, once it's put on a 'color card', he's in and he locks whatever it is into his brain.
So next time your child/friend/spouse, etc. tells you another fact about their obsession, go with them! Follow their obsession down the rabbit hole and find ways to use it to help increase concentration and focus on other areas of life.