For us their purposes have become clearer as we have gotten to know more about each alter over time. We've found that the littles don't so much serve a purpose for us now, but did so at the time of their creation. Some littles have continued to serve that purpose over time as it has been needed (and thus grown or aged) and others have remained locked away and frozen in that moment of time.
I can give a few examples of some of the purposes some of our alters serve...

C is 12 and is a front line protector.. meaning she fronts whenever "mild" stuff is going on or she senses that some of the little ones could get hurt (more emotional hurt, like rejection). C is out quite a lot.
AJ is an age slider from around 7-14. She is social and enjoys connecting with other people. She is protected from all hurt, so just knows light, happy things.
L is 3 and holds the deep yearning for a mother. She is closely protected by C.

For adults we have several that deal with different aspects of work. One is efficient, no-nonsense, get things done, and another deals with work relationships. We are mostly co-conscious with these alters, but it doesn't work back the other way. (Meaning they aren't very aware of the trauma stuff, but I am).
We have a mother alter who gave birth to our children and does most of the parenting. The work alters feel very little emotional connection to our children.
There are a couple of adult alters we still have very little awareness of, even though they are still active in our daily lives. I know what they deal with but I am not able to "think" about those things yet. Developing connections with these two is a current focus in our therapy.
We have an adult alter who holds depression, and another who holds sui feelings. They are the only feelings those alters know.
We have (or had?) an artist. In the past she has created artworks and given them away. From time to time people show us the artwork "I" have made for them, but I have no awareness, memory of or connection to the artwork at all. There have been paintings, sculptures, drawings, collages, all sorts of things that are actually quite good. I "know" they must have been done by some part of us but I still find the whole thing bizarre.
We have a very intelligent part that was responsible for us getting through university. Thank goodness. We couldn't have done that without her. She operates at work sometimes too.

Then we have a bunch of trauma holders that each dealt with a particular trauma (or type of trauma, or aspect of trauma) and are still stuck in time. If we come across "new" alters now it is most likely one of these. Most of them are little, but some are teens. I don't know how many of them there are.
Then we also have a bunch of inner helpers whose role is to soothe and care for those traumatized young ones. They don't "come out" as such, but soothe and care on the inside.
There are also protectors. I won't say much about them. They are fierce.

So in my experience each alter deals with a specific feeling or action or aspect of life. Its just that they're segregated into different self states rather than all operating under the same sense of self. It seems there's an alter for everything. We've got life covered.