Sameness in The Giver is a means of controlling the people in a few ways.  It is human to want to acquire more than you have, and if there are no choices of things to acquire, the temptation should at least theoretically be removed.  People who have no reason to strive are far easier to control than those who do strive. You meet their basic needs and they remain content. This sameness eliminates what might be class tensions in a normal society.  Everyone, with the exception of the elders, is the same, one class of people who are not competing with one another. Given one class of people who are the same, the governance of the society is far easier, one set of rules for everyone, with a blanket application for all, no need for any judgements or discretion. Sameness makes everyone in the society fungible in most ways, so that any woman can be designated to set up household with any man, and any two children can be chosen to be placed into that household. So there is an ease of administration for the ruling elders.   Sameness is also meant to remove envy, which prevents arguments and theft.  If everyone has the same possessions, the same sort of housing, and so on, there is no reason to envy anyone else, which keeps resentment and argument down and which removes any motive to take from others. This, too, makes the community easier to rule and control. 

What is "sameness," and why does Jonas's community decide to practice it?

You will need to find your own reasons why you feel the community adopts the policy of sameness. Readers may interpret the story and the actions of the elders in following that policy in different ways. Some will say Jonas's community is good because it provides a stable and comfortable lifestyle for all its residents; others will say the community is bad because it destroys any opportunity for individual self-expression.

''Sameness" reflects the efforts of the community members to create a society in which all citizens enjoy equality in all things. All residents are cared for by others within the community - infants by those whose job it is to be a nurturer, children by the man and woman to whom they have been given to create a family unit, the elderly by children who help out in the senior living facilities. All are assigned work based upon the abilities and interests they exhibit as youth. All are provided with food, clothing, shelter and transportation to give them the ability to function as contributing members of the community.

All are also controlled so as to be conforming members of the society. Language is expected to be exact and based on observable facts, with no unverifiable emotions allowed. Medication is routinely given, starting when an individual reaches adolescence, to suppress unacceptable thoughts and dreams. In the name of sameness, originality of thought or action is actively discouraged.