Using manipulatives creates the opportunity for children to talk about their learning. Talking about maths allows children to develop their understanding of how numbers relate to one another. Having a secure understanding of what numbers actually mean, enables children to develop a range of skills for calculating.
For example: understanding that 7 is 5 + 2 means that 15 + 7 can be much easier to solve; you can then do 15 + 5 = 20 then add 2 which is 22. Or for a more challenging question; understanding that 999 is 1000 - 1 means you can answer 7 x 999 by first doing 7 x 1000 and then subtracting 7 x 1 which is 6993.
Talking about maths also helps children to develop their understanding through convincing and reasoning, having to convince a friend that their method works shows how well a child understands what is happening in the calculation.