Monks are the ones dedicating their life to learning and practicing the Dhamma. Both learning and practicing correctly takes special circumstances and many years of perfecting it.
The Buddha made it necessary for these special beings to interact with laypeople, so they can also learn from the monks, and even practice if they ask for instructions. If this would not have been a rule, monks would not interact with laypeople. They would just keep to themselves, secluded from the world. Giving alms to a pure monk gives enormous merit to the layperson with forbearing positive consequences to their rounds in Samsara. It is a great opportunity, which is given to laypeople. They may even inspire a few to go forth and practice themselves, becoming enlightened. So it is a gift to us from the Buddha.