The purpose of punctuation is to provide useful signals to readers about the meaning that the writer wants a particular string of words to convey.
Because punctuation is not a part of spoken English, some experts on language seem disinclined to treat it as anything more than a style-based adjunct to orthography. It is true that the meaning of many written sentences is clear with or without particular choices of internal punctuation (commas, colons, semicolons, parentheses, and dashes, for example), which encourages people to imagine that punctuation is entirely arbitrary and idiosyncratic—purely a matter of style.
To the extent that including or omitting the commas after here and sand in your examples has little impact on the sense of those sentences, the choice to comma or not to comma isn't much of question; and those punctuation decisions do fall into the "purely a matter of style" category.
Nevertheless, any claim that punctuation is always just a matter of style ignores a couple of real-world considerations. First, if we accept that punctuation marks exist to provide meaningful signals, it is counterintuitive to suppose that they have no effect on sense or to argue that every punctuation decision is equally valid. In particular, readers are likely to experience annoyance when a writer repeatedly inserts punctuation marks in situations where the marks serve no practical purpose. That's why no one would happily read a page of text that put commas everywhere:
I'm, going, bowling, later, but, for, now, I'm, just, here, hanging, out, with, my, friends.
or that put them in random places where the marks fail to align with either the structural logic of the sentence or natural pauses in its course:
I'm going, bowling later but for, now I'm just here hanging, out with my friends.
Second, in some instances the more reasonably inferrable meaning of a sentence hinges on whether the writer chooses to include or omit a comma at a crucial location. That isn't the case with either of your example sentences, which indicates that the commas after here and sand in those two sentences are optional. But it happens more often than you might think. For example,
I'm looking for someone stopping at every bar in town to have a drink.
means I'm looking for someone who fits the description of a person who stops at every bar in town for a drink. But
I'm looking for someone, stopping at every bar in town to have a drink.
means I'm looking for someone, but in the course of my pursuit I stop in every bar in town to have a drink. It's a big difference. Likewise, in
I'm following a dog trying to catch a rabbit.
the lack of a comma strongly suggests that the dog is the one that wants to catch a rabbit and I'm just following the dog, whereas in
I'm following a dog, trying to catch a rabbit.
the sense of the sentence is that I'm trying to catch a rabbit and I'm following a dog in hopes of achieving that goal.
In both of my sets of examples, the comma signals a severance between the direct object (someone or a dog) and the following action (stopping or trying), which causes the action to point instead to the subject of the sentence (I, in both cases). If the comma is absent, the connection between someone and stopping and between dog and trying is unbroken and readers have reasonable grounds for inferring that the action is supposed to attach to the next-door direct object.
Most ambiguous sentences provide contextual clues that help readers figure out which meaning the writer had in mind. But internal punctuation, used sensibly and consistently, provides useful guidance, too. Clearly, punctuation isn't grammar, just as road signs aren't driving routes, but well-chosen punctuation can help you get your readers to the destination you want them to reach.