Observation periscope is just that...for observation. Especially for aircraft. It has a low magnification and wider field of view than the attack periscope.
The obs periscope is particularly useful for those times when you're "schnorkelling" since with the diesel's running you are essentially deaf and the hydrophones are useless.
Real-life Kaleun's would routinely use the obs scope before surfacing...especially in the latter war years when the air threat was much greater.