Not my case and planning to stay here, but...
1) Possibility to go back to Canada for people and their children any day and choose where to live in the future.
2) Possibility to send their kids for studies in Canada after they grow up with no hassle.
3) Probably easier to work in some other countries as canadians. In the US for a start.
4) This may not be life changing for an american citizen, but coming from an african country, the canadian passport is really a huge deal for international travel.
5) In case shit happens outside of Canada and the other country of citizenship, always good to have the support of the Canadian Embassy.
6) Easy to maintain property or business ties in Canada even while leaving abroad.
7) Probably various other reasons...
Also, the actual answer could simply be something like : "they had the right to apply, so why not ?"