As someone who has actually made a "look at my application" post and have had discussions related to my application, I think I have a couple answers:
1) Many people don't have anyone to show. They may not have made any connections with their professors during their undergrad, or never joined a research lab, or anything of that sort. The only people they know are their friends who are in the same boat they are. So they come here, to try and ask other people who have had applications, what they think of theirs.
2) Many people don't know how the process works, especially with so much conflicting information. On one hand some people say, "the GRE is a make it or break it deal". On the other hand, people say: "lots of people have gotten into top schools with low GRE scores", but those sources never specify why or what. Is it the GPA? Is it a good rec letter. And its difficult to answer because each persons application is different. So these sources don't really apply to everybodies application, and rather it must be approached as a per basis view. Each application must be looked at individually. For some, their experience and background is so impressive the GRE and their GPA don't matter. As someone who has done research on this, there is a lot of conflicting information, and anecdotal experience is best when it comes to these situations.
3) Often times (myself included), people come here for information on programs and a discussion. A very common example of this is: "Hey guys, I've done my research, and with my application it appears my chances don't look good. Should I still try? Do you think I'll have a chance?" This can be crucial when you're looking to apply to multiple schools and it costs a bit of money. Any bit of information can help. Maybe someone was in a similar situation you were in, and it worked out for them, so they tell you to go ahead and do it! Maybe someone on here can tell you a better option, a better school, that you may not have thought of or found.
4) Anxiety, depression, anger, disappointment, etc. We have all been through these things. Hell, I made an account for this sole reason. I did really bad on my first GRE, and I wanted to see how everyone else did, maybe my scores weren't all that terrible. I mean, from research I had come to the conclusion it was awful, but the community here really helped me realize that it wasn't the end of the world. My scores weren't that bad, and I still had a shot for going after a PhD program. This community is about helping spread information and support one another, and sometimes people just want to be told they have a chance, that they should go for it. For all the reasons stated above.
Anyways, I for one always answer to the best of my capabilities (I have recently joined, so haven't done that much) every question regarding applications that I see. Through reading a lot of personal anecdotal evidence on here, and looking at stuff online, I have a general understanding of how applications go, and can help at least try and reassure people, or at the very least give them some direction where to look or start looking.