I feel the need to be even more geeky here.
The cleansing of the way quote wasn’t unimportant.
To Paul, the hunter-seeker wanted him. It went for her because she moved. He used the opportunity to catch it, thereby saving both of their lives. To him, the matter was over. Mapes didn’t seem particularly bothered by how close she had just come to dying. Fremen are not afraid of dying. But, she was bothered by the fact that he had saved her life, putting the debt on her.
That comment in the book shows very early on that the Fremen are honorable. She was determined to pay back the life (water) debt. She also makes sure he knows that they pay back ALL debts. This was his first encounter with a Fremen and he learns very early on that it’s good to be on their good side, and BAD to be on their bad side.
Up to that point, all references to water had been the discussions between the Atreides and their people. They talked about how important it was on Arrakis but they always said it wouldn’t be a problem for the Atreides.
I think that exchange helped to make the water problem on Arrakis more real to Paul.
Water was important enough to her that she was willing to give up a valuable piece of information (not that the knowledge did them any good in the end). Where people in our world would have tried to sell that information for money, she did it to get rid of the water burden.
Additionally, thanks to the Missionaria Protectiva planting their propaganda, Mapes is already associating Jessica and Paul with their ledgends and knows perfectly well that Paul could be the Kwisatz Haderach, which means that he and his water could very well be more important than anyone knows. It’s bad enough having a water debt on your head. But, when it’s the water of someone your whole race has been waiting for for thousands of years, it could be an unbearable burden.
It still takes Paul a while to really understand the importance of water, shown when he initially refuses Jamis’ water. You’d think that mere minutes after this exchange, walking into the wet-planet conservatory would have had more of an impact on him. But, it helped to plant the seed.
For Jessica, she and Mapes had the discussion about the cry of the water sellers and the wasting of the body’s water when Jessica cut Mapes with the crysknife. She even noted the ultrafast coagulation of the blood - something in the Fremen evolution to help from water loss due to bleeding.
So, they both had a water talk with Mapes, the first Fremen they both meet.
It also helps for the reader. Soon after this exchange, Paul and Jessica are trapped out in the desert. The transition from the talk about the water debt immediately to the conservatory shows the reader, in very stark contrast, that there is a huge difference between what Paul and Jessica are used to, and what they have to look forward to. Paul and Jessica are both interested by the conservatory but their reactions are more, oh - cool, plants. A Fremen probably would have gone into a holy frenzy.