This function we rarely use because in general it leads to misunderstandings. If you are in a bakery and you want a certain type of cake you can of course say 'I want this one' and point with the finger to the cake you want. But normally this leads to a conversation like that.

A: I want this cake.
B: This one?
A: No, the one on the right.
B: This one?
A: No, below this one, the brown one.
B: Ah, the chocolate cake, why you didn' t say that before.

a) I want this bicycle.
b) I want the bicycle.
c) I want a bicycle.

The demonstrative adjective does not only point to something, but points out the singularity of something. Only a) expresses the idea that he doesn't want just any bicycle, but a very special one.

Have a look at this sentence.

I don' t like that.
I don' t know that.

That points to an idea. (I don' t like that he interrupted his studies to go on holidays.)

If you point to a single noun you use in general the pronoun it.

A: Do you like the house?
B: Yes, I like it.

With demonstrative pronouns you can describe temporal or local closeness / distance.

At that time we realized that there was a problem.
At these days, it's all about creativity.