To attract huge investments to fund manned missions it is necessary to provide an economic return
Currently the plan to make space exploration physically viable is to provide an economic return. This can be accomplished by either importing mineral resources of the planet or implementing factories to take advantage of planet resource. However if questionable that the huge investments requires to initiate mechanism will provide a return in a reasonable amount of time and in the case whether this can be done with a fully drone mission.
Return on space expedition can not only need to be monetary, but can be scientific advances that can fall-out also on everyday life A classical example is the topic of climate changes. So far the only full model we have is the Earth itself. Studying planets like Mars or Venus could provide other references
To convey money from investors an economic return should be provided.