Use of some and a

[diplomacy]

Why do we use a instead of some in these sentences although the nouns are uncountable ?

1/ I would like a coffee please.
2/ Can I have a beer please.

And here why do we use some?
1/ I want some water if you don't mind.
2/ There is some coffee if you like.

[5jj]

Why do we use 'a' instead of 'some' in these sentences, although the nouns are uncountable ?

1/ I would like a coffee please. ......2/ Can I have a beer please?
We are thinking of a (cup of) coffee or a (glass of) beer.

And here why do we use 'some'?

1/ I want some water if you don't mind. ......2/ There is some coffee if you like.
We are thinking of the drink rather than the container.

5