I'm going to take a chance here. Don't take my opinion as the final word.
Animals tend to live in an ever-expanding present. They generally don't plan too far ahead, except for some apes.
For humans, on the other hand, past, present and future are tightly linked. We can't talk about the present without at least thinking what went on before in the past.
Also, we can't plan for the future without using the past and present situation as a basis.
It is impossible for us to talk about past, present and future alone and separate without at least giving some thought to other times - especially when it comes to our own personal experience.
He also says (not in your quote) in the next line that time is not redeemable - which means to me the obvious - that time which has gone by is lost forever.
Since I never studied T.S. Eliot (except for "Hollow Men"), anybody may feel free to rip my analysis apart on this.