The story
Formed because of the rhyme between the names of the letters E and C, /iː/ and /siː/. The "rule" was invented in the 19th century.
Etymology from Wiktionary, CC BY-SA 4.0.
i before e, except after c
Today it means: In English words that contain the letters I and E together, the I tends to come before the E (ie), except in cases when the two are preceded by the letter C, in which case the E comes before the I (cei). Full definition →