as good as nonexistent; practically useless; too inadequate to count
Used when something nominally exists, but is so small, weak, incomplete, inconvenient, or useless that it does not serve its expected purpose. Most natural in the fixed-feeling phrase あってないようなもの.
この部屋の収納は小さすぎて、あってないようなものだ。
The storage in this room is so small that it is practically useless.
説明書はあるけれど、内容が古くてあってない
There is a manual, but the information is outdated, so it is as good as nonexistent.
忙しい日は休憩時間なんてあってないようなものだ。
On busy days, break time is practically nonexistent.
役に立たない is a plain way to say 'useless', whereas あって無い is a more idiomatic expression implying something exists but doesn't count as sufficient or functional.
A transparent expression from ある/有る 'to exist' plus ない/無い 'not exist'; the phrase conveys that something exists formally but not in any useful or meaningful sense.