They are defining their territory and we don't need to so convenience we do it all at once. Dogs will only generally do small urine deposits when on a walk but in the garden will urinate normally.

It is like writing a letter:. You don't have to write it all at once. Pee is a means of communication to dogs.
But I do not agree with you. Not all have to empty the bladder at one opportunity.

It's not that dogs CAN pee small amounts - but that they WANT to. In fact, they NEED to.

In the wild it was a vital survival technique.

Dogs do pee simply to empty their bladders at times but pee strategically at other times to lay down a 'scent' or a marker.

A dog has a sense of smell up to 2,000 times stronger than ours - so they live in a vastly magnified world of smells - not suprising when you look at the length of a German Shepherd's or a Bloodhound's nose and think of all those extra nasal receptors lining it.

Each dog's pee has a distinctive (at least to another dog) smell that is as clear as an autograph.

Dogs can easily detect precisely when and precisely which dog has peed where up to a week or more after the event. This gives them a dramatically clear idea of how the neighbourhood is divided up - who rules where etc.

So peeing is marking territory and saying to other dogs 'this is my area so watch out'.

Equally they can tell by the freshness of the pee-points just how often a particular dog has crossed their territory in the last few days and even at what times of day they are usually out and about.

That is useful for avoiding confrontation.

It explains why dogs are forever running around with their noses on the ground sniffing walls, trees, lamp posts etc. It's vital local knowledge.

It also explains their 'charming' habit of sniffing other dogs' (and people's) crotches. They are simply checking out their 'signature tune' and storing it for future reference.

Thank god we have evolved other methods of getting to know each other.