There are two possible interpretations.
On the one hand it can be read as an observation that a professional often doesn't maintain the same standards and practices at home as they do on the job. So for example, a mechanic whose tool bench at the shop/studio is meticulously organized and well-maintained: every tool has a place and every place has a tool, and whenever the professional uses those tools, he always finishes the job by cleaning and polishing every tool, and replacing those which are damaged. Contrast this with home, where the workbench is a morass of random tools all scattered about with no organizational paradigm, many of them rusted, duct-taped together, or covered in oil. In this case, the adage refers to the fact that when it's a matter of one's livelihood or a service performed for other people, a professional tends to be much more careful and concerned about quality and method; a sense of professional pride plays a major role, whereas at home they're only really accountable to themself and their spouse/children, and in this case the only real concern is "getting the job done."
The other interpretation is more of an economic critique: the profits earned from the practice of a service doesn't afford the professional the material capacity to enjoy those same services at home. On the job I earn wages by transforming the finest ingredients into intricate, sumptuous, artistically refined and expressive dishes to delight and satisfy clientele of the highest order. However, the money I come home with at the end of the day only grants me enough to purchase a stale loaf of bread and some low-quality cheddar cheese. I can spend months in the restaurant kitchen producing countless delicious variations of dishes using saffron, but I myself haven't a single thread of the stuff in my pantry. That sort of trope.
I like either interpretation, because they're essentially making the same structural argument/observation: that society has hewed a clear division between the economic and the domestic (pun fully intended) spheres of life, and that for the vast majority of people, that division is necessarily, lamentably, unequal.