The s key in normal mode deletes the character under the cursor and immediately enters insert mode. This is one keystroke shorter than xi, which does the same thing. But s is also repeatable with the . operator!
Let’s try it in the editor. The mathematical expressions are missing spaces between the digits and the operators. E.g. we’ve got 1+3, but we really want to write 1 + 3. Let’s use the s key to fix these up. Place your cursor on a + symbol, then type s + in normal mode. You can then move to any other + and hit . to repeat!