Apparently the intention is that they are somehow better in battle combined than as individual robots. But why are they better combined?
The only advantages I can think of are:
- Strongly physical strength eg a huge punch. That is better in a one-on-one, hand-to-hand fight but usually I would imagine six robots fighting is better than one.
- More powerful gun. This is not always the case of course (eg Superion uses Silverbolt's gun) but for some (eg Devastator) they can carry one huge weapon. Again, this is better against one opponent but usually I would think that six guns are better than one.
- Redundancy; that is having a whole robot as an arm makes it less vulnerable to damage than having a "normal" smaller robot arm. But the downside is that the combined form is one huge target!
I'm looking (of course) for the in-universe reason.
(The out-of-universe reasons are obvious: it looks cool and it sells more toys when you have to collect a set!)