What is the main benefit of sass or scss. How is it better than boostrap and tailwind. Everybody is just advicing me to learn sass because they see my projevt using only css with no framework to design a full website.

Speeds up your productivity.
You can extend similar properties, use custom functions(mixins), while loops, if-else statements etc.

Take this forEach equivalent in sass

$spaces: (1, 2, 3, 4, 5);
@each $space in $spaces {
.#{$space}-div {
margin: $space * 2px;
}
}


There's also for loops

@for $x from 1 through 10 {
.-#{$x}-style {
padding: $x + 4px;
}
}

You can imagine how when you add JavaScript to these you can create complex animations.