Compare the present form and will + (verb)

The present form simply refers to the present-future, and is often used for general statements.

We use will when there are options or possibilities.
The future is often unknown, so we consider possibilities and choose what we think will happen (we make predictions).
We also consider options and choose what we will do (we make decisions).

present simple tense timeline - in general - many times

It rains a lot in summer. (general statement – many times, every summer)
I don’t eat lunch at work. (general statement – many times)
Do you buy her flowers? (general statement – many times)

future simple tense timeline - will: modal verb - decision or prediction - there are options or possibilities

It’ll rain a lot in summer. (considering possibilities, what we think will happen – a prediction, next summer)
I won’t eat lunch at work today, I’ll go to the park. (considering options – what I choose to do – a decision)
Will you buy her flowers for her birthday? (considering options – what you choose to do – a decision)

However, we don’t need to add will to talk about the future.

We use the present form for fixed events in the future (these things can’t be changed). We say or know when they happen.

We use will + (verb) when we consider options or possibilities and a choice is  made(the future is unknown, so we say what we think will happen).

present simple tense timeline - schedule - fixed future

The package arrives tomorrow. (A fixed future event – scheduled)
We don’t have a game next week. (the schedule)
What time do you start work on Friday? (your schedule)

future simple tense timeline - will: modal verb - decision or prediction - there are options or possibilities

The package will arrive tomorrow. (The package could possibly arrive on a different day, the speaker is saying what they think.)
We won’t have a game next week. (It was possible, but it won’t happen. Maybe there was a change of plan, or someone cancelled.)
What time will you start work on Friday? (Considering that there are several times you may possibly start work)

When we talk about the present, we often don’t think about other possibilities and simply use the present form.

But sometimes when we talk about the present, we use will + (verb) if there are other possibilities.

(phone ringing) It’s my husband. (A fact. Maybe I looked at the screen and saw who was calling, or he told me he’d call at this time.)

(phone ringing) It’ll be my husband. (It could possibly be someone else, I’m saying who I think it is.)

Also, when we make general statements, we can use will if there are other possibilities.

Water boils at a lower temperature at higher altitudes. (a general fact)
Accidents happen. (In general, we accept this.)

Water will boil at a lower temperature at higher altitudes. (most people know that water boils at 100°C, but using will emphasizes that there are other possibilities)
Accidents will happen. (In general, we know it is not possible to stop all accidents from happening.)

Sometimes the meaning of will changes the meaning a lot.

If she doesn’t call, we’ll go without her. (simply, she doesn’t call, maybe she forgot)

If she won’t call, we’ll go without her. (she considers her options and decides not to call – she refuses to call us)