Condensation often gathers in the bottom of agar slants. why is it important in this exercise to limit condensation on plates but not on slant

Because condensation on the slant effects the bacterial growth in the test tube.

Explanation: When agar is solidify in the test tube tilted, it takes a shape of slide on one side of the tube which is known as slat. Slant is used instead of petri dishes because they provide more surface area for growth. Water in the slat can invade the test tube, making agar unstable and the bacterial growth on the slat will be affected by moisture in bottom of slant. In petri dishes, it is no big problem of moisture condensation.