What Mars does in a chart
Mars is the part of the chart that answers one question:How do I go after what I want? It describes drive and desire — not events, and not outcomes.
What Cancer does to it
Cancer is a water sign, so it moves on feeling and atmosphere. It is also cardinal, which means it starts things. Put those together with Mars and you get acts protectively — protective, tidal, homing, applied specifically to drive and desire.
Cancer is ruled by Moon, so this placement tends to be coloured by emotional needs as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.