为什么有的词后面要加ing

语法需要,如：
固定结构,enjoy doing sth,look forward to doing sth;
时态结构：进行时：Look,the boying is playing with his mum。I was cooking when you called yesterday evening。
动词加ing作形容词：That was really exciting news。He's an interesting man。
动词作主语时,须变成加ing的动名词：Walking is good exercise。Playing a kite is fun。