It is often used to express:
* Regret: "Si tu savais ce que j'ai vécu, tu comprendrais" (If you knew what I went through, you would understand).
* Frustration: "Si tu savais à quel point je suis impatient!" (If you knew how impatient I am!).
* Sympathy: "Si tu savais ce qu'il traverse" (If you knew what he's going through).
* Curiosity: "Si tu savais ce qu'il se passe" (If you knew what's going on).
The phrase can be used in a variety of contexts, and its meaning can change depending on the tone and the situation.