Niraj - if you don't declare daqObj as a global variable, then it will be a local variable. When your function "completes" (after calling start(...), then all local variables will be deleted/destoyed. If daqObj is local, then it will be destroyed and will stop trying to acquire data so this is probably why "nothing happens". But if you declare it as a global variable then it will "exist" outside of the function (so that other functions could access/share this variable) and so the data acquisition will work.
You mention that you are using App Designer, so why not make the daqObj a property/member of the GUI so that you don't have to declare this as global? That way, you can control from within the GUI when to start and stop the acquisition.