Do crew need visas when landing at airports in countries other than their own?
It all depends on the laws of the country that the crew is visiting.  Some do and some don't.  Many do unless they have a reciprocal agreement with the pilot's country, in which case they don't.

This would be a great question for a specific country pair, or even if you want to know the laws about a specific country (in which case we could give the actual requirements), but the best answer for a general question like this is "it depends".

In some cases, the crew isn't even required to clear customs unless they are leaving airport property.  Other times they have to clear as soon as they land.

Part of qualifying for a route with the airlines is making sure that you have the required visas.  Having two passports help a lot in getting multiple visas, because they can keep flying internationally while the visa application is in process.