Since a travel agent is involved for a Europe trip, I'm going to guess there's some unusual / unique aspect to this booking? I'd want to know the purpose of the him/her wanting the information.

The times I've needed third parties (travel agents) to get involved with a trip have involved them having a specific relationship with an event, excursion operator, or other in-country specialty services. It's possible some of those people need passport information to provide services to foreigners.

It's also possible the TA just wants it to make 100% sure that your ticketing info and predeparture information is loaded correctly. I can see them wanting this for group travel (another reason people sometimes use TAs), although I haven't personally done one of these trips.