How does smarttab actually works?

Question

Yes, I'm aware of what :h smarttab says, but I still cannot figure it out.

" My vimrc
set backspace=indent,eol,start
set tabstop=8 " I always keep this 8
set shiftwidth=4
set softtabstop=-1 " To use the width of `shiftwidth`
set expandtab
set smarttab

When on, a <Tab> in front of a line inserts blanks according to shiftwidth. tabstop or softtabstop is used in other places. A <BS> will delete a shiftwidth worth of space at the start of the line.

When off, a <Tab> always inserts blanks according to tabstop or softtabstop. shiftwidth is only used for shifting text left or right shift-left-right.

But I observe the same behavior even if smarttab is off. Pressing the Tab key inserts 4 spaces, and pressing the Backspace deletes 4 spaces. Why does it happen? What is the reason to use smarttab then?

Answer 1

What is the reason to use smarttab then?

As it is written in help, when smarttab is off, softtabstop is used everywhere including line start. However, if softtabstop is equal to -1 then it is set to shiftwidth anyway. Hence there'll be no difference in setting smarttab on or off.

In other words, smarttab is only meaningful if both set expandtab and softtabstop differs from shiftwidth. But normally it's not the case and we don't need smarttab.