點解西裝褸既手袖邊會有幾粒袖口鈕

There are usually four buttons on suits, 3 buttons on sportcoats or blazers, and 1 or 2 buttons on country coats.
These buttons and buttonholes are meant to be functioning. When in need, you can unbutton them and roll up your jacket sleeves.
There are many stories regarding the origin and the tradition. One of them is about surgeons, in the old times, having the need to roll up their sleeves while operating (remember surgeons and doctors travel from places to places, not always in uniforms operating in big hospitals like nowadays).
Nowadays, if you get your suits made at tailors, these buttons are always functional buttons - meaning, the button holes are open. In ready-made suits from cheaper brands to Armani, the buttonholes are closed and the buttons on sleeves are merely decorations.
Here is a picture of one of my jackets with functioning sleeve buttons.
http://img.photobucket.com/albums/v514/naturlaut/DSC09896.jpg