Here reference is to one of God's great commandment, which is a means for purifying man's soul and his society. It is a general law for Muslim in all ages, that a small part of their wealth and property is to be spent in some charitable purposes for the sake of God.
Through alms giving, the corruptions and mischiefs that arise from poverty among men and their society can be removed or at least reduced. So also by such mutual assistances, man can improve the economical affairs of their society. Therefore, the alms may purify people, and pave the way for the apostles, and spiritual teachers, to bring them up-up in ranks much higher than that of animals.
Allah has bidden His Messenger in this verse to pray for those who pay their due alms-tax! This implies that, we always have to give our thanks and gratitude, to those who do something good, even though their good deed be their duty, or an obligatory fulfilment. Such a behavior will encourage people to continue doing good.
The verb, ``TAKE ALMS'' which is in imperative mood, imply that the Islamic governor should not wait for people to come and bring their alms-tax. He may send his functionary delegate to collect their alms.