Why do dogs sniff when they need to urinate and what would be the exact reason they sniff?

They are marking their territory. They will either smell another

dog's urine and urinate over that or, they will mark their own

territory such as the backyard/frontyard, etc.

it tells them that that is their territory. if one dog, wolf,

... sees anouther dog mark its territory onto theirs, it will

usually start a fight. think about this example to help you

understand if you get a glass of milk and you take a drink, that

cup is your territory until you are done. lets say your little

brother or sister comes into the room and takes a drink out of your

cup. you get mad because that cup is your territory. it is the same

thing with dogs, its normal.

What dogs smell, whether it's urine or another dog's butt, tells

the dog just about everything about the other dog. Dogs have

millions more scent receptors than do humans and they way the

process smells is very different than we do. For dogs, scents are

comparable, almost, to how we perceive things by sight. The smell

of another dogs urine tells them everything about that dog -- it's

sex, whether it's in season, whether the other dog is healthy or

not, etcetera. Once they determine what the scent is telling them,

they pee over it to mark the territory as their own or just to let

anyone else coming by that they were there, thus leaving their own

information-filled scent.