A person gets divorced, a friend that has never been married says "I understand"
A grieving mourner at the funeral home has someone tell them "I understand"
A person with health problems hears from a healthy person "I understand"
Even Christians say this at times. No, we don't understand at all.
Pray with the person, talk to them, encourage them with scripture, but don't claim to understand how or what they feel. when you don't.
From personal experience I can tell you it gets frustrating and even insulting.