Diagnosing some diseases , detecting landmines , detecting food safety, detecting tracks(like dogs), analysis of biocompatiblity of couples(through pheromons),improved robotic chefs can all achieved by smelling.

In general , smelling , i.e. analyzing the volatile compounds materials emit can teach us some things about those materials and what processes they are going through.Those seem like usefull capabilities for many industries.

"Pizza is ready" ( Throw away badly burned pizza) ( Fix constants in script) ( Put new pizza in oven.) Repeat from top.

I really can not see why anyone does not want a computer with a sense of smell.

Might make an interesting authentication mechanism....

I don't know about PCs, but for mobiles it seems like any sensor you can put in it, someone will figure out an app that can use it.