[*]when you want a same day boarding pass on co-StarAlliance (promissing seamless travelling ...) member UA but have a first segment and check-in with LH[*]when, on arrival, you must provide proof to immigrations/costums that you have a valid return ticket (Egypt, sometimes USA, Singapore, etc.)[*]when (on UA) you are not able to present the credit card the e-ticket was bought with (lost/replaced credit card, ticket bought with another credit card of your wife/friend/company/etc.)