Like the previous answer to this question stated, food is used in different ways, but mostly as a symbol of attachment and status.
The foods most well known for their cultivated taste, the crumpets and the cucumber sandwiches that Algernon eats after promising them to his aunt, are a symbol of detachment from his responsibility towards her.
The huge meals in which he indulges and never pays for in restaurants are also a symbol of detachment from his responsibilities towards society in general.
The requirement of a meal when he went to meet Cecily against Earnest's will was his way of making a bond and attaching Cecily to him.
The showdown between Cecily and Gwendolyn over their respective "Earnests" had its insults thrown around in the form of food:
Cecily's offering of cake and sugar lumps for the tea denoted her lack of sophistication according to Gwendolyn, who was devoted to bread and butter, and unsweetened tea: She finds these more "fashionable" and even "honorable." And the offer that Cecily made of cake and sugar was met with haughtiness as ammo for insults.
The final food mention in the story shows Algernon and Earnest eating THE VERY FOOD that Cecily and Gwendolyn were fighting over. This may be another symbol of their overall detachment to THEM. We know that Oscar Wilde was not only homosexual but he did show a specific disdain for marriage and the idea of it, in general. In his plays, he likes to present the idea of a man ignoring their nagging female companions.
Perhaps when Algernon and Earnest ate the all-important cake and tea and bread and butter, they were basically telling off the very angry women, and indulging instead of the company of food, used here as a symbol of detachment.
The role of food is connected to the presence of social mores and conventions. The central food-related scene is of course the tea service between Cecile and Gwendolyn. The social niceties of the tea ceremony allow the women to channel their anger and anxiety into the the ritual, and to accuse each other of inadequacies of social status is their habits regarding the taking of afternoon tea. As well, one of the Earnest characters has an appreciation for "muffins" and their presence in an average tea setting. This reveals this character to be one who is sensual in nature and attached to his personal rituals and habits, thereby showing him to be someone not necessarily suited for marriage, which is of course one major social theme of he play.