What it teaches
Multiplication and division inside 10 × 10, asked both ways round. 7 × 6 and 42 ÷ 6 are one fact, and a child who only ever meets the multiplication half has learned half of it.
The tables are the last piece of arithmetic that has to be known rather than worked out. Everything after them — long multiplication, fractions, ratio — assumes they are there.
What to look for
Look at the parent screen for the facts that keep returning. There are usually only four or five of them, and they are almost always in the 6, 7 and 8 tables. Those five facts are the whole job — the other ninety-five are already known.
Saying the fact out loud together, twice, when it comes up beats ten more minutes of random practice.