I ask them too because I teach alg, these problems become more complicated as the year moves on and they need to write their steps down because there are multiple steps. The ones that have been doing it in their heads for years suddenly cant seem to do that anymore and find themselves in a tricky situation because they don't know how to show their work.
This is usually my go-to website for guided math (math workshop) ideas. Tabitha and Chloe are active bloggers who have a wealth of good ideas that a teacher in any grade can modify.