Unitary operations are a subset of CPTP operations. You can think of a CPTP operation as the description of a unitary over a larger system.
The advantage of using CPTP maps is that you increase the generality of your statement. Think about, for example, a proof of the no-cloning theorem. People usually start talking about the input state, the target state, and an ancilla, so you want to achieve some rotation
|0\rangle|0\rangle|0\rangle\mapsto |0\rangle|0\rangle|a\rangle,\qquad|1\rangle|0\rangle|0\rangle\mapsto |1\rangle|1\rangle|b\rangle
where the third system is any dimension. We then assume that the process is unitary over that whole space. This is equivalent to allowing a CPTP map on the first two systems. Why don't we just completely ignore the third system and only assume a unitary on the first two? Well, what use would our no-cloning theorem be if it turned out that by adding some extra ancillas, our proof was completely invalidated and that cloning were possible? You want to perform your proof using the full generality of what's available, which means not making any assumptions about availability of ancillas, for example.