Physical activity is not only important for your child's health - it will help them cope with the sheer physicality of interacting with twenty children on the playground. Bumps and shoves are inevitable, so make sure your child has lots of physical play to develop gross motor skills too.
By the time your child is ready for kindergarten or school, they should be able to recognize their own name and other simple written words. The sounds of each letter of the alphabet should be familiar to your child, and they should understand the principle of reading from left to right, which way to hold a book, and possibly even be starting to read three and four-letter words.
Worksheets also, when used properly, provide both the students and parent / tutor immediate feedback as to the child's progress. This means they can be used to point out areas where the student needs further reinforcement.
Almost everything you do with your child can be turned into an opportunity to learn - and worksheets can give you the guidance you need to find those opportunities.