Radar was being used for the first time, to help planes to land.
Why didn't they use (Radar was used for the first time, to help planes to land.)
We use the progressive in the passive voice in the same types of circumstances in which we use the progressive in the active voice. The progressive allows us to talk about what is or was happening at a certain time or time period.
The comma before "to help planes land" tells us that the main point of the sentence is that it was the first time radar was used at all. The fact that it was to help planes land is a parenthetical detail.
I don't know whether you have access to the context (the sentences that come before the sentence you have asked about), but it is surely discussing a particular time or time period. At that time, radar was being used for the first time.
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