Counting by 2s, 3s, 5s and up to 12s, with one number missing from the sequence: 6, 12, 18, 24, ___, 36. Every sequence starts at the number being counted by, so it is the times table written out in order.
That is exactly what makes skip counting worth practising - it is multiplication before the word arrives. A student who can count by sixes to 72 already knows the six times table; these sheets make the sequence visible so the gap has to be reasoned to rather than recited past.
Counting in steps larger than one: 3, 6, 9, 12 instead of 1, 2, 3, 4. It is the foundation of multiplication and of reading a clock in fives.
These sheets use 2 through 12. Each worksheet mixes several, so a student has to work out the step from the sequence rather than being told it.
Because a gap at the end can be filled by adding the step to the last number without noticing the pattern. A gap in the middle has to be reached from both sides, which is the skill.
Second and third grade, usually alongside or just before the times tables.