The kepler space telescope is searching for extrasolar planets by the transit method. it is necessary for kepler to photometrically monitor a large number of stars because

The Kepler Space Telescope is searching for extrasolar planets by the transit method. It is necessary for Kepler to photometrically monitor a large number of stars because increase the probability to see a transit.

Explanation:

Photometry is the study of the intensivity of light radiated from a particular object.

In the other hand, the transit method consists in the measured of the dimming on the brightness of a star when a planet is passing in front of it, as long as the star, the planet and the detector (in this case the Kepler Telescope) are in the same line of sign.  

However, that transit has a short duration. So it is necessary that the Kepler Telescope monitorates the brightness of several stars each thirty minutes in order to increase the probability of detection of a transit.