"Hereby grant" means the grant is (supposedly) immediately effective even for future-arising rights — and thus would take precedence (again, supposedly) over an agreement to grant the same rights in the future. [0]
(In the late oughts, this principle resulted in the biotech company Roche Molecular becoming a part-owner of a Stanford patent, because a Stanford researcher signed a "visitor NDA" with Roche that included present-assignment language, whereas the researcher's previous agreement with Stanford included only future-assignment language. The Stanford-Roche lawsuit on that subject went all the way to the U.S. Supreme Court.)
[0] https://toedtclassnotes.site44.com/Notes-on-Contract-Draftin...
Not necessarily — in some circumstances, the law might not recognize a present-day grant of an interest that doesn't exist now but might come into being in the future. (Cf. the Rule Against Perpetuities. [1])
The "hereby grants and agrees to grant" language is a fallback requirement — belt and suspenders, if you will.