Intelligent people ask intelligent questions.
Indeed it is true that if a particular setup can profit $2 per day there is seemingly no sensible reason to lease it out for $1 a day when you could have the $2 a day yourself - it would make more economic sense to lease it out for $3 a day.
From another perspective, someone who wants to mine can forgo the initial outlay for mining equipment and hire mining power for a fee. It is possibly not true, also considering the cost of running the machine, that leasing mining power may be more cost effective than purchasing and housing mining equipment yourself - most high-power mining equipment is also quite loud. It is still to consider that the equipment you lease may have lower costs for power and cooling than you could provide.