Why do they "Cosplay"?

from some funny articles in local papers, which I spotted lately..

C2E2 Cosplay in the Chicago St. Patrick's Day Parade 
 http://www.chicagonow.com/geek-girl-chicago/2013/03/st-patricks-day-parade/

..It's funny  .St.Patricks' day is not just a day "for the Irish descents" to remember their ethnicity..  but now,..its a day for everyone for turning Irish by cosplaying?..

..I guess maybe I saw the greeny Chicago River's picture before..., but I thought it was just an water pollution!!

St. Patrick's Day GEEKend: They Might Be Giants, Fanfiction Live, and More
...St. Patrick's Day is huge in Chicago thanks to beer, parades, and the famous green dye in the Chicago River. Did you know that the dye is still a "secret formula?" SOMEONE has to know what's in there- I can't believe they'd let that much gunk get dumped, otherwise! I'll be marching for C2E2 in the parade on Saturday dressed as Rose Tyler from Doctor Who. It's gonna be freezing but fun!
 http://www.chicagonow.com/geek-girl-chicago/2013/03/st-patricks-day-they-might-be-giants/  

Chicago River Green: St. Patrick's Day 2013 Celebrations Continue Around The Windy City (PHOTO)  http://www.huffingtonpost.com/2013/03/16/chicago-river-green-st-pa_n_2891531.html
 Lely I
..and it fits to the neighborhood's character, after all?

At Uncommon Ground, Egyptian Musicians Play With a 'Message for the World' 
 ..Two Egyptian musicians, who protested in Tahrir Square during their country's 2011 revolution, traveled to the city's North Side to play at the Edgewater Uncommon Ground location in a show called "The Arab Spring Concert." 
Nour Ashour and Ahmed Derbala are a long way from their homes in Alexandria, Egypt. They had their first American meal — a "tasty" burger — Tuesday at St. Andrews Inn, said 22-year-old Derbala. On Thursday, they plan to satisfy their dream of going to a Chicago Bulls game. The two friends were invited to play in the show by its creator, Allie Deaver-Petchenik, the director of Classical Revolution Chicago. The musician's collective works to bring classical music to coffee shops, restaurants and small venues in Chicago's neighborhoods. Deaver-Petchenik said she met Ashour and Derbala while traveling last summer in Egypt and Tunisia. 
 "During my time [traveling]," she said. "I got to work with some pretty spectacular musicians." They jammed together there and stayed in touch. And then when the opportunity arose to bring them to Chicago, the Egyptians made it happen... http://www.dnainfo.com/chicago/20130321/edgewater/at-uncommon-ground-egyptian-musicians-play-with-message-for-world#ixzz2OBbTbuNs