As Molliepops says, there are a lot of calories in bread/rice/couscous/potato etc. For myself, I like rice/couscous/potato. But in terms of allocating my daily calories, they an be inconvenient. 100g dry couscous = 367 kcal or so. That's a fair chunk of my daily allocation. (Admittedly that is a generous portion). Sometimes it annoys me slightly that I can make a spaghetti bolognaise with fairly low calorie 'mince' (I'm vegetarian) and sauce/veggies mix, but then I add the spaghetti - oh dear. Same applies for chillies/curries and rice, etc.
If the cauliflower 'rice' worked, then it'll be an option for me for some days when I want to eat something like frosted and filled chocolate cake after dinner. I'd have to save calories from elsewhere, and substituting cauliflower 'rice' for real rice would help achieve that.
Oh right. Yeah I'm aware that in some ways you don't get much rice/pasta for the calories. But in my case I'd much rather have a proper meal of rice and veg (or whatever) than 'save' the calories to eat a chocolate cake.
A chocolate cake wouldn't fill me up at all so rice is a much better option.