The need for a sump pump is quite obvious, when the rain brings water into the basement floor. Of course, the real solution is to consider placing a corrogated drain system around the perimeter of the basement, as well as replacing those leaking gutters or properly grading the property.
In the short term, the pump can hold back water damage that could become extensive quickly. The most effective type is the one that sits in a 24"deep X 12"wide hole in the lowest plane of the basement. The hole is also lined with an insertable plastic bucket with seep holes that allow subsurface water to fill it so that a floating switch can activate the pump.
Also, it is advisable to purchase a battery backup system that is charged by normal house current at all times. The sump pump is not a permanent solution, but an affordable, intermediate one. Standing water in a basement is not only bad for health, it can damage a water heater or furnace. We can see the sump pump as a life boat on a sinking ship.