It's also useful even after they learn to talk, because you're still gonna have a hard time understanding them until they're well into their second year.
Signs my kid has learned and used, without a lot of prompting from us:
"All done"
"Please" (also means 'yes' -- "Do you want jelly?" "please")
"help"
"water"
"milk"
"food"
Absolutely, my primary motivation initially was to enable our first to tell us when he needed the potty [I'm a kook, we start potty from 5-6 months], though initially for the first it was mostly telling us he wanted milk/food or saw a light/bird.
We read a lot, and always do sign and speech together. Eldest is in Secondary School (UK) now; if it retarded his speech/language skills as some claim it might then it didn't matter because he and middle child are highly literate (several years advanced in reading age).
Mum, Dad (none of my kids bothered with that sign much), food, drink, milk (both kinds), sleep, tired, hungry, more, finished .. all useful starter words.
From 2+ we start faltering with signed vocab so sign (a modified and simplified BSL somewhat of our own) tends to naturally give way to speech -- I mean what's the sign for gazelle, or pneumatic drill, or to say "it tastes bad" rather than just that they "don't like" whatever new food we're trying to get them to try ...
>It sounds bizarre to many Westerners. But for parents in places like India, China, and East Africa (deVries and deVries 1977; Boucke 2002), the traditional potty training age is early infancy. In these societies, parents learn to recognize their babies’ body signals and to use these signals to anticipate when their babies eliminate.
>When the infant is ready to go, the parent holds him over a sink, bowl, toilet, or the open ground. As the infant voids, the parent makes a characteristic sound or gesture. The baby learns to associate this parental sign with voiding, and, eventually, the parental sign becomes an invitation to void. When the baby feels the urge to go, he learns to hold back for a brief time until his parent gives him the “all clear."
Primarily in my "system" (!) the idea was to offer the potty at nappy changing times, first thing in morning, after nap, before bed etc. - using sign and vocal prompts (that idea is used in Elimination Communication) - and go crazy over-the-top with praise for anything they do in the potty (but not lying, at 2 youngest is like "big wee" when there's only a spot, because he wants to please; "good try!", Always applaud trying!). At 6mo of age you have to hold them up. But my reasoning was habituation, familiarity, and just reduction in messy nappies.
First child was in terry-nappies, and compostables at night. Latest toddler is in compostables all the time -- terry definitely seem (on my low-number study) to help associate weeing and the feeling of it starting. Nappy free time in the garden or at the beach will help them associate the feeling with the actions too.
Sign, we knew some BSL so started with simplified versions of signs (removed all hand shapes, so Mum is a flat hand on side of head rather than an M-hand; exaggerate movements, make sure signs are differentiated). Just speak and do signs for anything they'd usefully learn - food, more, mum-milk, sleep, potty perhaps as an opening gambit. From 7-9 months they should be able to pick up at least a dozen signs and by 10 months be signing back; obviously that's assuming normal development and consistent use by caregivers.
Most sign books use ASL, which can be confusing if you go to a class and all the signs are different.
The potty and sign go together, giving then a way to communicate their need and a way to greater comfort - I imagine they play off each other.
Signs my kid has learned and used, without a lot of prompting from us: "All done" "Please" (also means 'yes' -- "Do you want jelly?" "please") "help" "water" "milk" "food"