请教转成小写的代码是起到什么作用？

以下是代码全文：

public class MapTest {
public static void main(String[] args) {
    String str = "fdg+avAdc  bs5dDa9c-dfs";
    String s = getCharCount(str);
    System.out.println(s);
}

public static String getCharCount(String str){
    char[] chs = str.toCharArray();

    Map<Character,Integer> map = new TreeMap<Character,Integer>();

    for(int i = 0;i<chs.length;i++){
        if(!(chs[i]>='a'&& chs[i]<='z'||chs[i]>='A'&&chs[i]<='Z'))
            if(!(Character.toLowerCase(chs[i])>='a' && Character.toLowerCase(chs[i])<'z'))
                continue;

        Integer value = map.get(chs[i]);

        int count = 0;

        if(value!=null){
            count = value+1;
        }
        count++;
        map.put(chs[i], count);
    }
    return mapToString(map);
}

private static String mapToString(Map<Character,Integer> map){
    StringBuilder sb = new StringBuilder();
    Iterator<Character> it = map.keySet().iterator();

    while(it.hasNext()){
        Character key = it.next();
        Integer value = map.get(key);

        sb.append(key+"("+value+")");

    }

    return sb.toString();
} 
}