What's the purpose of the capacitor in this circuit?
As mentioned in comments C701 is a DC blocking capacitor. The 5.6 volt bias voltage is part of a DC servo-loop to keep the final output saw-tooth at a specific DC level. C701 combined with a low resistance input does modify the square wave input, but as shown it is the edge that this circuit syncs to.
Without the DC blocking capacitor, which does have a high-pass filter effect as well, this circuit loses filtering of the square wave and loses the DC isolation it needs. C701 allows the square wave input to have most any DC reference, including zero volts, without upsetting the performance of the sweep generator.
Before being put into use this circuit had to pass military testing for accuracy and long-term stability, and a servo-loop is a good way to do that. Part of the &quot;Hold-off&quot; circuit feeds back part of the saw-tooth to the same 5.6 volt node. This prevents re-triggering of the sweep generator in the middle of a sweep.
The upper section of the sweep generator inserts a blanking pulse so the retrace of the sweep is blanked out, similar to analog TV.