But then, you could still take selfies without a selfie stick, so buying on is just not having anything better to do with your money, in this case useless=IRRELEVANCE
You could also do math without a use of a calculator. But the calculator makes math easier. And the the selfie stick makes taking a self portrait easier and with more options. Yes you could take a selfie without the use of a selfie stick. But just because you can doesn't make the selfie stick useless. The use of a selfie stick is to extend the camera beyond the reach of ones arms. Wide angle shots, more subjects in the frame. Not having the look of holding the camera because one of your arms is extended out of the shot. Make the picture you took of yourself look like someone else was holding the camera. So it does 'achieve the intended purpose'.
As far as "useless=IRRELEVANCE". I don't understand what you're trying to do here. Two words, with different definitions don't equal each other.
useless: not fulfilling or not expected to achieve the intended purpose or desired outcome
irrelevance: the quality or state of being
irrelevant
irrelevant: not connected with or
relevant to something
relevant: closely connected or appropriate to the matter at hand
"useless=pointless" I and agree with. "irrelevance=insignificance" I can understand. But "useless=irrelevance" It just don't work.