First, the meaning of "sea" is an expanse of salt water that covers....haha JK. OK now seriously, the meaning of "sea" is "to be" or "is". It is not only used when uncertain or doubting. It is used in reference to future events (with some exceptions). So you couldn't say "El sea fuerte" (He is strong).Here are some examples: "Ya quiero que sea viernes."(I want it to be friday already.")
Asegurate que sea jugo y no refresco."( Make sure its juice and soda.") "Quiere que su hijo sea doctor."(He wants his son to be a doctor."). The example you used is correct as well. So it can show uncertainty but also a hope/desire for something or an assurance for something. Frequently used with Ojalá (hopefully) and Asegúrate (make sure) at the beginning of the sentence.
The exception: "sea" is also used when commanding someone/something to NOT be something. Example: "No sea(s) grosero." ("Don't be rude.")
Too much info maybe but hopefully it make sense and helps.