Possible Powers-In chapter 126, Yūko explained why Maru and Moro are required to stay inside the shop. The shop itself is not seen by anyone unless they are fated to do business with Yūko. Maru and Moro's concentration is what is keeping the shop in existence. As a side-effect, they have long deep bouts of sleep.
They help out around the shop, but mostly act as the cheering squad. They almost always speak in unison, or close to it. Watanuki seems to get along with them most of the time, at least when they are not dragging him into one of Yūko's plans.
It is noted that Maru and Moro's full names are both words which mean "exposing oneself in public" – similar to "flashing" and "streaking" in English, and something that is quite possibly a pun on the 'xxx' part of xxxHOLiC. Even though the 'xxx' are intended to act as a variable in the title, their appearance resembles the 'XXX' which is synonymous with pornography. Given the target seinen audience, this double meaning is probably intentional.
There are very few instances that Maru and Moro are in serious scenes. In the parasite angel chapter, Watanuki invited them to join him in the festival, however Maru and Moro said in unison (in a rather dazed look) "those without souls cannot leave this place... for their bodies will disintegrate."
They also show some concern not only for Yūko but also for Watanuki despite the fact they always annoy him. In volume 6 of the manga, when Watanuki came home from visiting the "lady in the park", Maru and Moro immediately sensed that something was wrong with Watanuki. When Watanuki finally collapsed, the two dragged Watanuki as far as the store's gates, saying "This is as far as we can go" before tossing Watanuki out to a waiting Dōmeki. In the anime, Watanuki also shows signs of slight affection to the two; in episode 18, when Dōmeki read a letter to Watanuki written by Mokona and Maru/Moro, Watanuki was somewhat pleased to see that they thought of him.