As this dictionary says can is used with the verbs see, hear, feel, taste, and smell, and with verbs connected with thinking, to mean that someone sees something, hears something etc. For exapmple:
Here they are – I can see their car.
I can’t understand why you’re so upset.
Can you smell something burning?
But what if I omit can from the sentences, for example:
Here they are – I see their car.
I don't understand why you’re so upset.
Do you smell something burning?
Is there is any difference in meaning between the sentences? If there is, then help me to see one please.