As long as those pikes are, when they are leveled they are not the best protection against arrows and javelins. So the first ranks needed some added protection. Not much but it all helps.
Further, the buckler was not as small as the Levy Pikemen's shield, it was in fact around 60cm in diameter, which is not large but certainly not too small either. So I don't know if I would call it a buckler.
The phalangites were not always used as pikemen, but also as swordsmen and javelineers when the terrain was not for big blocks of pikes. In such cases a shield would be a serious help.
Lastly the shield was strapped to the forearm but it also had a strap up and around the shoulder and neck (just like a shoulderbag), making certain the shield kept facing outwards as well as helping the left arm keeping the pike leveled. Without the strap the pike would become quite heavy for the left arm after a while, but with it a lot of the weight was transferred to the back and neck.