Not a single person is able to explain why it matters.
Here, let's try this: For the sake of argument, let's suppose that Arbery was not getting water in that house.
Now, finish this sentence: "Given that Arbery was not fetching water, we can then rationally infer __________________"
I can think of two ways to answer why it matters.
1) I don't know, but the various media outlets that reported the lawyer's speculation that he might be getting water.
2) It has been said that the only reason one would have for visiting a construction site multiple times is to steal something. However, there are other reasons that he may have been on the construction site.
a) He was studying to be an electrcian (that's true, or at least it was said in this thread) and wanted to observe the house wiring.
b) He knew that the house was a source of water to use while running.
So, from a legal perspective, there is no real relevance to the question of what he was doing at the construction site. Nevertheless, public perception, including jury response to arguments, is colored by things which are not strictly relevant. If the public perception is that Arbery was a burglar, it will be harder to obtain a conviction of the McMichaels. I am not saying it should be harder, but it will be. Providing an explanation for his presence on the construction site that does not involve wrongdoing would make it more likely that a jury could be persuaded to convict his killer, and his killer's dad.
To put it in terms of your fill in the blank formulation, we can "logically" infer very little if we could determine that he was not getting water. There's the trivial "he was doing something else", but no other logical inferences. However, not all inferences that people make are logical. Giving an explanation of why he was there that does not involve unlawful activity would be persuasive to a lot of people, even if it wouldn't pass muster in the world of formal logic.
Finally, there's a variant on answer 1 above. It is, "I don't know if or why it matters. I saw a new story about it, so I said something about it,"
ETA: And my point in all this is that sometimes it's painful to participate in forum discussions, especially on controversial topics, because any deviations from the party line story are frequently met with accusations that must be on "the other side". Sometimes people speculate on things like whether it would be logical for Arbery to jog in that neighborhood, or what is the significance of Greg McMichael not giving a street name, or why someone visited a construction site, and whether a possible explanation is a plausible explanation. Just because someone brings up one thing from a news story that they don't think is correct does not mean that they support the side that would benefit if the thing in question were true.