Maisy has a good point about how little sustenance carriages require but I should think they do need maintenance even if not in use and it might not require much money but it might take some time to avoid having a rusted out hulk after a winter of resting in one place. If Mrs. D keeps her carriage it would necessitate someone (their one male servant) keep it in working order even if it is not being used and IMO that does not sound like a very practical use for your lone male servant who must be very busy indeed.

Perhaps Mrs. Dashwood had thoughts of keeping her carriage and then using post horses to visit friends and relatives. It was common to use post horses to rest your own on a trip (see link below to an L&T archived thread) so maybe post horses could be brought to the carriage to begin a trip—I do not know how feasible that is—just an idea. An advantage to having your own carriage might be comfort if yours is nicer than those used for post and you would be assured that a carriage was available when you wanted it—I do not know the likelihood of horses being available when a carriage is not because I do not know the common ratio of horses to carriages at these stops. Another idea is that Mrs. D thought she would someday be able to afford horses again.