Immutable String Buffers

Why is this telling me to concatenate small strings to the ends of large ones, but not in reverse, and then concatenate all remaining strings in the table?
http://www.lua.org/pil/11.6.html

Wouldn't it just be easier to put all the strings in a table then concatenate them all at once, as in the following?

function filetostring(file)
    local t == {}
    for line in io.lines(file)
        if line == "" then
            break
        end
        table.insert(t, line .. '\n')
   return table.concatenate(x)
end

I'm confused as fuck.

>>4
If I have to concatenate every line together and I'm doing it line by line why not just do it at once.

I can see why this is wrong.
 -- WARNING: bad code ahead!!
    local buff = ""
    for line in io.lines() do
    buff = buff .. line .. "\n"
    end

But how is this any better then what I wrote? If you need to allocate a new piece of memory every time you assign a string to a variable, then why wouldn't this be worse?
function addString (stack, s)
    table.insert(stack, s)    -- push 's' into the the stack
    for i=table.getn(stack)-1, 1, -1 do
        if string.len(stack[i]) > string.len(stack[i+1]) then
            break
        end
        stack[i] = stack[i] .. table.remove(stack)
    end
end