The buttons traditionally are for decoration – at least now. Back in the day, these buttons worked. They were for rolling up your suit jacket sleeve for labor and to prevent soiling of you clothing. Unlike now, laundry was difficult to do and wasn’t as efficient, so men would roll up their sleeves on their coats in an effort to keep clean. These buttons on jackets are referred to as “Surgeon’s cuffs.” This is because surgeons in the field or where ever they worked would just roll up their sleeves and get to work.
Once in awhile, you can find working buttons form “off-the-rack” suit jackets, however it is rare. This modification is typically reserved for bespoke (custom) tailored jackets. Just FYI – people that wear bespoke jackets with working buttons, usually have the top one (furthest to thumb) undone to show they are wearing custom.
There you go – just in case you were wondering.