Well that is definately a strategy used. Typically one lane will consist of a 'babysit' type hero. Who tries to harass the other team and heal up the ally in the lane. While his ally would be a 'carry' hero, who needs all the money he can get. Carries want to get the last hits to gain money and exp, while a babysitter does not need as much money.
Early game, you cant dish out a lot of damage at once, so it is best to try to gain the upper hand by outleveling your opponent, by denying. Then when you reach a higher level (even as early as level 3) people begin to harass more. While still trying to gain the money from last hits.
Early game, last hits and denies are really important. A creep typically gives between 30-50 gold, while a hero gives around 200. If there are 5 creeps per wave, that is still between 150 - 250 gold + the experience.
PS: forgot to fully describe denying, yea it can only happen when a ally creep has less than half health.
PPS: PETER LEARN IN SCHOOL