Why is 一緒に needed when it's already clear two people will be together?


A father is talking to his kid about their plans tomorrow and says,

> けんしんくん、お父さんと一緒に買いに行きましょうか？

Why not:

> けんしんくん、お父さんと買いに行きましょうか？

(The sentence is supposed to say, "Kenshin, would you like to go to shopping
with dad?" Obviously, not a one-to-one translation)

Aren't both sentences the same? I can't see the second sentence **not**
relaying the idea of "together". So what does the 一緒に do exactly? It seems, to
me, that the sentence doesn't really need it.




At the coffee shop after practice, 行こうか is a perfectly good sentence if the
highest ranking person says it. Everyone will get up and leave. In other
situations, _do you want to go home_ is very different from _do you want to go
home with me._ Japanese can drop the _you_ , but still needs the 一緒 (but not
the me/us/them). I remember hearing a lot of 一緒に without names or pronouns. I
know this is an example sentence, but I wonder if since 一緒に is so often
required in this kind of question, it isn't automatically used and then names
are added for emphasis. By 'then added', I mean some kind of mental process
that assembles the sentence before speaking, not final word order.

