According to Young, "perspective" is derived from the fact that differentiated people, by virtue of their varied social position, have "have different experience, history, and social knowledge derived from that positioning." She conceptualizes perspective as a "starting point" for how we think about political affairs, but not an end -- it sets up our vantage point, but does not determine what we decide to look at or how we interpret what we see.
Here, Young explains why -- notwithstanding the fact that everyone, by virtue of their unique background and experiences, has a different perspective -- it makes sense to think about perspective in terms of group positioning. She also stresses that saying that people might share a perspective (or at least a similarly positioned one) is not the same thing as saying they will reach the same normative conclusions about what principles flow from that, or what their political interests are.
To be sure, each person has his or her own irreducible group history which gives him or her unique social knowledge and perspective. We must avoid, however, the sort of individualism that would conclude from this fact that any talk of structured social positions and group-defined social location is wrong, incoherent, or useless. It makes sense to say that non-professional working-class people have predictable vulnerabilities and opportunities because of their position in the occupational structure. The idea of perspective is meant to capture that sensibility of group-positioned experience without specifying unified content to what the perspective sees. The social positioning produced by relation to other structural positions and by the social processes that issue in unintended consequences only provide a background and perspective in terms of which particular social events and issues are interpreted; they do not make the interpretation. So we can well find different persons with a similar social perspective giving different interpretations of an issue. Perspective is an approach to looking at social events, which conditions but does not determine what one sees.
Iris Marion Young, Inclusion and Democracy (Oxford: Oxford UP, 2000), 138-39.
So, for example, all gay people in America (save Massachusetts) share a "position" in that none can marry the partner of their choice. That does not mean that they all share the same ideas of what moral principles should be brought to bear on that issue (what Young calls "opinions"), or what they imagine the effect or importance that reality has on their life prospects (what Young calls "interests"). Two gay people could start from their relatively similar perspective, and radically differ on what moral principles they apply to it, and/or what interests they imagine they have regarding gay marriage. But nonetheless, they operate from a different standpoint than straight people, who necessarily don't "see" the problem of marriage inequality in the same way (even if they adopt the same interests or opinions on the matter as individual gay people).
Perspective matters because it can bring agenda items to the democratic table that might otherwise go un- or under-noticed, or offer new ways of conceptualizing a problem that the dominant perspective may be less likely to see. To talk about needing to include the gay perspective, or the Black perspective, is not to say that we are committed to a particular conception of their interests or ideologies. Elsewhere in the book, Young cites the example of the Pittsburgh Courier, an African-American newspaper. Though its news stories deal with a wide array of topics, and its editorial pages host political opinions ranging from right-wing libertarianism to left-wing socialism, it still identifiably comes from an "African-American perspective." It adopts an outlook that operates from the vantage point of where African-Americans reside in our social hierarchy, but still recognizes that people from that position can have widely variable interests and opinions.
Because people experience many social problems and controversies in different ways, it is important to insure that relevant groups are represented in discussion of those issues, even if we (rightly) assume that not every member of that group will have the same idea of how to respond to these issues.
UPDATE: An excellent example I just thought of illustrating this is my good friend, Clarence Thomas. Like many Blacks, Thomas interacted with the institution of affirmative action. He shares the position of his fellow African-Americans who also have reaped whatever benefits and hindrances that are the upshot of that institution -- particularly, the feeling by some Whites that he is less qualified as a result of being a "beneficiary" of it. Now, as is clear, that can lead to a variety of opinions on the subject -- Thomas considers AA actively harmful and wishes to abolish it, many other Blacks believe the problem is the White prejudice against AA graduates (flowing out anti-Black prejudice generally), not the program itself. The perspective, in other words, clearly doesn't determine one's ultimate opinion. But even granting that, it would still be absurd to the extreme to say that we should debate the question of affirmative action without consulting/representing those whose social position has given them this specific perspective on the matter.