It's not a dim question at all.
The most important issue is whether the baby has some sort of developmental difficulties or not,then exactly how much language they have.Only a salt can really ascertain this,and then not always terribly accurately.(I am a salt btw)
Language can be roughly divided into comprehension (understanding) and expression (which is usually mainly speech but not always.Noises and facial expressions also count amongst other stuff.)
Generally,babies understand first then start to speak so logically,by helping them to understand better you will be helping booost overall language development.
Signing helps as it provides extra visual clues as to the meaning of words (especially as the signing takes place with normal speech).
This can be helpful as a lot of children salts see have poor auditory processing. (Ie They can hear,but can't make sense of what they hear.Rather like you or I listening to a badly tuned radio)
Now,if the child sees lots of signs being used,we hope that she susses out that this is a pretty effective and easy (less neurologically demanding than speech)way to communicate and tries out signs herself.
A lot of parents worry that signing will hinder their child form speaking but research disproves this.It acts rather like scaffold...an extra support.It's a win win situation. If the child will never speak then they do at least have an effective structured method of communication.
If they do learn to speak,the signs will give them a 'leg up' and will be discarded when no longer needed.
Hmmm,this probably wasn't exactly what you wanted to know...