In some cases, it is possible to use "will" in "if" sentences to imply

A) willingness

If you'll just hold the door open for me a moment, I can take this table out to the kitchen.

B) obstinate persistence

If you will keep all the windows shut, of course you'll get headaches

C) if the "if" action is after the main action, will (or an equivalent expression indicating the Future) is used in the if clause

If aspirins will cure it, I'll take a couple tonight.

Please check Wikipedia and lingua.org and a page from Berkeley

Edit 1 Per @Brett Reynolds and @Peter Shor

As far as I can see, "will" may first impose a change in the order of actions and then "both "will" and "order of action" contribute to meaning. The contribution depends on context and may imply "a willingness", "a persistence" "a wish", "a prediction" e.t.c. Please compare:

I will give money if you will do it.

 

I will give money if you do it.

In some cases, it is possible to use "will" in "if" sentences to imply

A) willingness

If you'll just hold the door open for me a moment, I can take this table out to the kitchen.

B) obstinate persistence

If you will keep all the windows shut, of course you'll get headaches

C) if the "if" action is after the main action, will (or an equivalent expression indicating the Future) is used in the if clause

If aspirins will cure it, I'll take a couple tonight.

Please check Wikipedia and lingua.org and a page from Berkeley

Edit 1 Per @Brett Reynolds and @Peter Shor

As far as I can see, "will" may first impose a change in the order of actions and then "both "will" and "order of action" contribute to meaning. The contribution depends on context and may imply "a willingness", "a persistence" "a wish", "a prediction" e.t.c. Please compare:

I will give money if you will do it.

I will give money if you do it.

In some cases, it is possible to use "will" in "if" sentences to imply

A) willingness

If you'll just hold the door open for me a moment, I can take this table out to the kitchen.

B) obstinate persistence

If you will keep all the windows shut, of course you'll get headaches

C) if the "if" action is after the main action, will (or an equivalent expression indicating the Future) is used in the if clause

If aspirins will cure it, I'll take a couple tonight.

Please check Wikipedia and lingua.org and a page from Berkeley

In some cases, it is possible to use "will" in "if" sentences to imply

A) willingness

If you'll just hold the door open for me a moment, I can take this table out to the kitchen.

B) obstinate persistence

If you will keep all the windows shut, of course you'll get headaches

C) if the "if" action is after the main action, will (or an equivalent expression indicating the Future) is used in the if clause

If aspirins will cure it, I'll take a couple tonight.

Please check Wikipedia and lingua.org and a page from Berkeley

Edit 1 Per @Brett Reynolds and @Peter Shor

As far as I can see, "will" may first impose a change in the order of actions and then "both "will" and "order of action" contribute to meaning. The contribution depends on context and may imply "a willingness", "a persistence" "a wish", "a prediction" e.t.c. Please compare:

I will give money if you will do it.

I will give money if you do it.