Some preemergent weed killers are mixed with fertilizer so two jobs can be done at once. But don't confuse them with weed and feed fertilizer products. Weed and feeds only work when it's warmer and the weeds are up and growing. Remember with any fertilizer or weed killer, more is NOT better. And sometimes a few weeds in the lawn isn't worth the spread of chemicals.
Some lawns are thin in places.
Not much grass and a lot of soil is showing. Top dress those areas with new grass seed. Seeding works best when our nights are above 40 degrees. That's usually in late April. Sprinkle seed over the bare spots.