Some languages follow a scheme where every main clause must contain a marker, which is built right into the verbal conjugation system, to indicate whether the action is set in the past, present, or future (from the speaker's point of view). Since this is true of many Indo‐European languages – and especially since it's true of Latin – it is taken absolutely for granted in traditional schoolbook grammar, and people often find it hard to grasp that there could be alternatives, let alone that they might have advantages.
However, only about half of all languages grammaticalise tense in anything like this fashion; and you don't need to go far to find one that doesn't work like Latin. English does have grammatically compulsory tense marking on verbs, but it's only a two‐way distinction between past and non‐past; we don't strictly speaking have a “future tense”. Seriously, don't listen to those classics‐crazed schoolteachers! I will leave is just a modal construction – parallel to I can/should/must/might leave – with uses that include referring to a future time (compare I leave tomorrow). That doesn't make it a grammatical future tense any more than o table! is a first‐declension feminine singular noun in the vocative case.
It's already routine in English to use a variety of present‐tense constructions when talking about the future, as in we're leaving soon, because it's going to start raining; and it's easy to imagine a minor variant of English applying the same sort of logic for past events – we're recently leaving, because the rain is coming from stopping. Tense inflections are anything but essential! Indeed, quite a lot of the time they're somewhere between useless and actively misleading:
- Plutonium is an actinide – a timeless scientific fact presented as if it might become false at any moment.
- Time is a dimension – an abstract truth that's logically incapable of changing over time.
- This dormant volcano erupts once every million years – remote past plus remote future equals present tense.
- I could give it up tomorrow if I wanted – imaginary future scenario, but marked with the past tense inflection.
- Suddenly the Klingons attacked – fiction uses past tense by convention regardless of dateline.
- I was born in 1967 – the tense marking is redundant, even if this is a time travel story.
In English you might plausibly argue that the simple non‐past form is just the unmarked default, not a present tense, but that certainly isn't true for Esperanto, where all the tenses are equally elaborately inflected; and Esperanto forces us to put these inflections on verbs that would be better off without them, treating sentences like the above as if they needed to be pinned to a timeframe just as much as my alarm clock is ringing.
Some of the languages completely lacking in tense inflections are weakly inflected in general, while others instead focus on signposting the subtly different concept of “aspect”. Either way, this doesn't mean that they are incapable of specifying whether a thing happened in the past, present, or future; it just means that the question “when?” is treated the same way as others like “where/why/how often?”, and answered where relevant by means of adverbs or auxiliary verbs or the like.
As with all mandatory inflectional markers, replacing it with some sort of optional non‐inflectional modifier makes it easy to do things that Esperanto currently can't:
- asserting the truth value of a statement to be invariant over all of eternity (or just over as much of it as we care about) without inconsistently tying this assertion to a single particular moment in time;
- deliberately leaving the dateline vague because it's unknown, or irrelevant, or confidential;
- leaving it unstated because it's obvious, perhaps because a temporal context has already been established;
- throwing the emphasis onto it when it's important;
- putting modifiers on the time markers themselves (to give forms such as a “future superlative”);
- applying exactly the same optional time markers to adjectives or nouns;
- turning those markers into an open set that can be extended if you absolutely need to.
(If you think that last is never going to happen, ask a physicist. In an Einsteinian universe, events in a distant galaxy are not necessarily part of my past, present, or future!)