Until a more in-depth answer arrives - yes, it is. I'll answer based mainly on what I was taught in several seminars and lectures at my university (materials would be in german, so not really helpful here), but did manage to find an article in English as a source that specifically targets that point. Yes, it was to attach the arms.

Adding the arms - and also the head - separately proved practical for mainly two reasons. First, you could create a fairly standard body of an armoured man (or wearing a toga and so on), which would then be customized by adding limbs in the desired position and/or holding specific items, and a portrait head of the person in question.

Secondly, it took care of structural problems of marble - extended limbs would be prone to break off because of their own weight when the statue was made from one block. Earlier statues are often seen having marble support beams struts for extended arms (which, ironically, in some cases broke but the arms held, seen here on the Vatican Apoxymenos for example http://www.vroma.org/images/mcmanus_images/apoxymenos.jpg) With the use of these attached arms, the weight would be distributed along the anchor dowel between arm and torso, adding greatly to stability.

I apologize if my English is less than perfect, not my first language and it is really early around here.

Edit: Fixed wording, spelling, added source.