This famous poem has been the subject of much analysis and speculation. As with Eliot's other poems, there are times when the language seems very simple and straightforward, and other times when the language is intentionally symbolic, metaphorical, or allegorical.
The first thing that this line achieves is to set the poem firmly in a moment in time: the present. It may not be April at the time the poet is speaking, but the thought and sentiment is in the present tense, indicating a thought process happening concurrent with the poem's writing. The statement also confers a certain surety and authority; the speaker has no hesitation in naming April the cruellest month.
We also know that the speaker is attempting a sort of omniscient authority; there is a suggestion that they have experienced every month over many years and have reached an informed conclusion that April is the cruellest one. The vicissitudes of spring weather, as some readers assume, may be indicated. But rather, I think the beauty of spring is a source of pain for the speaker, who continues in the poem to lament the decline of civilization in a number of subtle and symbolic ways. The ability of humans to appreciate beauty, and their awareness of spring as a recurring occasion of beauty, are ironic counterpoints to the many examples of pain, brutality, oppression, guilt and regret described in the poem's later stanzas. April is cruel, because beauty is cruel; beauty is cruel, because humans cannot refrain from cruelty even in a beautiful world.
As well, the vegetative cycle mentioned in a previous answer is an interesting correlating factor: the idea that cycles of human behavior repeat themselves, just as the seasons do. Earlier in human history, people appeased the gods of nature by practicing human sacrifice, believing they would enjoy greater abundance of crops and animals if they offered human lives to the gods. The perpetuation of this cycle, and the reminder in spring that the time of harvest will come in the autumn, seems to be a theme being explored here. Rituals of sacrifice were practiced in the autumn, in preparation for the dormancy and food shortages of winter. April's cruelty lies in the awareness of the necessity of death as a balance to life; early humans sought to manipulate and control this balance, possibly to redress the damage done by human settlement, agriculture and animal husbandry—all subjects Eliot also touched upon in the poem.