Why do we say "Practice what you preach"?

Meaning:

If you tell somebody how to behave, make sure you follow the same advice yourself!

Background:

This expression is first recorded in the works of Roman playwright Titus Maccius Plautus (c. 254–184 BC). Plautus is believed to have written at least 52 comedies, mostly adapted from works by Greek playwrights which he reworked to appeal to Roman audiences. The twenty that have survived are among the earliest surviving intact works in Latin literature.

Plautus' works have inspired many playwrights since, including Shakespeare (for example, The Comedy of Errors combines the plots of two plays by Plautus), and Stephen Sondheim's musical A Funny Thing Happened on the way to the Forum, which is inspired by Plautus' works, in particular Pseudolus, Miles Glorisus and Mostellaria.

Plautus' epitah reads

Since Plautus is dead, Comedy mourns,
Deserted is the stage; then Laughter, Jest and Wit,
And Melody's countless numbers all together wept.

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