While reading the first manga of 「3月のライオン」, I came across an interesting usage of kanji for eggs. Until then, I've only seen 卵 used for eggs, or its readings using hiragana or katakana. The protagonist of the story received a text message on his phone, that asked him to drop-by the convenience story on the way to the sender's home, in order to buy some groceries. 「あと、玉子も！！」was at the end of it.
I'm wondering why these kanji were used and if there are general guidelines when to use one or the other.