Idioms can be strange (The elephant in the room), random (He's one sandwich short of a picnic) or downright weird/disturbing (Don't throw the baby with the bathwater / Killing several birds with one stone).
They sometimes involve body parts: to cost an arm and a leg, or one eye in Italian (costare l'occhio della testa) and both eyes in French (coûter les yeux de la tête). French and Italian feel the need to remind us that eyes belong to the head, oddly enough, and the French version is more costly than the Italian one!
Many a French idiom involves food and cooking, unsurprisingly: a turnip is a bad movie (don't ask!), splitting the pear in half (couper la poire en deux) means to compromise, and you don't get your hands dirty, but rather stick them in the dough (mettre la main à la pâte).
Some are identical from one language to the next:
To sell your soul to the devil / Vendre son âme au diable / Vendere l’anima al diavolo
To ask for the moon / Demander la lune / Chiedere la luna
To have one's head in the clouds / Avoir la tête dans les nuages / Avere la testa fra le nuvole
Idioms are a fun and easy way to not only learn new vocabulary, but also to gain confidence while speaking. You might feel a bit odd the first time you're using one in the language you're learning, precisely because it's the first time, but in fact, it's will allow you to connect more easily with native speakers through shared experience and humour.
In other words, learning idioms is a no-brainer!
PS: If you know why Bob is meant to be your uncle and Bertha your aunt (I've yet to come across an actual explanation for this one), don't hesitate to get in touch!
Don't hesitate to send me your questions.