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This answer explains that some Atlas payloads were covered in an air coditioning blanket before launch.
I can imagine that the payload has some thermal requirements, but a blanket looks lust plain strange from today's perspective. Why a blanket was used instead of just supplying conditioned air straight into the fairing?
The blanket was a temporary solution. Later Agena launches have an AC supply to the fairing itself. This was Thor-Agena 11, a reconnaissance satellite launch:
I haven't found a source that gives a rationale for this choice. But keep in mind this was the start of the age of spaceflight, and many things were tried out - and replaced if/when a better solution was found.
It may have been something as simple as an initial batch of fairings that was produced without AC ports, heating inside the fairing turned out to be a problem and a quick, temporary solution was rigged up until new fairings could be produced.
