The first mishnah in masechet Shabbat categorizes different cases of transferring an object between reshut harabim and reshut hayachid . It teaches these cases through the example of a homeowner providing for a poor person at his door. Why does it use this case?
The simplest answer (of the rambam and others) is that it is a practical choice for communication, shortening the description, instead of repeating “the person in the public domain”, “the person in the private domain”, it can say “the pauper”, “the homeowner”.
A second possibility (of the bartenura and others) is that it chose a case of Tzedaka to incidentally teaches us that even though it is a mitzvah, one still is prohibited to transfer on Shabbat.
Is there a third possibility?
The mishnah often explains through concrete material cases rather than stating the principle directly. Is this only as a language shortcut (also useful for memory), or to include extra laws through implication? Or does it conceptually set up the topic in Torah shebaal peh terms. Is it important for us to think of hilchot shabbat through the lens of tzedaka? Does this provide the framework for transitioning from torah shebikhtav to torah shebaal peh? Is the mishnah a systematic presention, not based on abstract category (like the mishneh Torah), but rather an initiation into a network of thought, bringing the reader into the beit midrash? If so, how should the mishnah be read?
 Reshut harabim and reshut hayachid are usually translated as public domain and private domain, but it isn’t a distinction of ownership, rather definitions of space. Reshut Harabim is a place which is available for use by all of humanity, such as a forest or marketplace, it is a place which lends itself to human use without restriction. Reshut hayachid is a place with the inside separated from the outside, such as with a wall, (including a city!). It is a place which is delineated for restricted human use, there are those who are in, and those who are out. There are other places which aren’t set up for human use at all, such as the sea (which we enter as a foreign element), some of which enter the prohibition of carrying, but only Rabbinically.