谁懂啊 turns a personal reaction into an invitation: recognize this feeling with me. The speaker has usually just shared the tiny disaster, lucky break, or awkward moment that makes the emotion clear, so this is not a literal request for an explanation.
The detail before the phrase matters. A bare 谁懂啊 gives the listener almost nothing; after a specific everyday mishap, it asks for sympathy and lets the speaker be a little dramatic. Friends might answer 我懂, share a similar experience, tease gently, or move straight to helping.
