Children's muscles need developing before any formal writing is comfortable for children, so it is vitally important to build them up. These activities will really help!
If your child's writing is faint, looks uncomfortable, a bit spidery (Not clean, straight or curved lines that look like what they want it to look like) you should be doing these sorts of activities before encouraging your child to write.
If your child is at the early stages of writing and is beginning to form letters etc, then these activities alongside writing, will help to build up stamina and support their writing development moving forward.
If your child is a confident writer, forms all letters correctly and enjoys writing, then continue to practise more formal writing. However, these activities will still help develop how long they can write for comfortably...so do the ones they enjoy!