Same with a lawn. Certain plants like clover have advantages like not requiring much nitrogen. Many grasses are very fussy and need good soil to thrive. But the grass finds its advantage in that it can grow taller than clover and therefore block out the sun. As the grass blade gets longer it is able to collect more sun (surface area grows) and establish deeper roots and block out the clover even better.

Many people use a variety of chemicals to hold weeds at bay. But a well watered lawn that is regularly fortified with organic nitrogen and kept at a decent length will more or less prevent most weeds from establishing. Many people cut their lawn way too short, removing the grasses advantage.