the basic aim of makaton is to facilitate communication.
We use it with dd1 and dd2 and no doubt no3 when they arrive.
My eldest(4yo) has down syndrome and the makaton was a godsend to enable her to communicate. We started around the 2yo mark and to see how far she has come in such a short time with limited SALT is fab.
We started with a few signs - more, drink, eat, sleep - the important basics. The thing to remember is to say the word as well - don't just sign. So you start signing and saying and then your child will start signing - usually an approximation of the sign. Then after a while the sign gets clearer, then you can get the spoken word and sign and eventually the spoken word without the sign. That is how it has worked for dd1.
Although she has a lot of language now - signing is still very effective. I use it to introduce new ideas/concepts/ themes etc and very powerful when she is not listening to me iyswim - to be able to reinforce with a sign. As she is such a copy cat and very visual - signing was a godsend. It limits her frustation, allows her to communicate with us and others.
For some people depending on their impairment - they may never speak but are able to use signing to communicate.
I taught dd2 (NT 2yo) a few signs and she just picked up so much from watching us interact with dd1 and attending therapy with dd1.
I think it really helped my girls not go through that frustration of not being able to communicate when they wanted and of me not being understood. FWIW - dd1 understands far more than she can communicate at times but I firmly believe that MAKATON gave her a voice.