As above

Some people use them when you have a septic system. If the kitchen is in the basement or lower level, may be used to pump up to the sewer or septic. You are sure it's not the disposer?

If you can find a way to run the sink gravity drain (and vent) that does not use a pump/electricity then I would advise you to do that if you purchase the place. Pumps break (and cost in electricity and repairs), so if there is another way, I would search for it and see if you can make the change.

If there is no way to run the pipe to a gravity drain then you now know why they have the pump installed.

Mark