Different opinion from me. I would say no, unless you have a very restricted or unbalanced diet, you absolutely don't need one.
I speak as someone who took multivits for years, swallowing (along with the capsules) the standard marketing line that it's a sort of 'insurance', in case you're missing something. When I stopped taking them, I experienced a modest increase in my disposable ncome
and no other effects.
I now wouldn't touch multivits with a barge-pole. Some very large, well-designed studies have found increased
cancer rates from quite a modest increase in supplemental vitamin A. Supplemental anti-oxidants looked like a good bet for preventing cancer (which is why the researchers looked at them), but when the results were in, they showed otherwise. These harmful effects were shown in smokers, so it might not be safe to extrapolate to non-smokers, but I think it shows that they're not necessarily benign substances/
I think it would be alarmist to call vitamin supplements dangerous. BUT I would go with the Cochrane Review (the best plain-language science/medicine reviews around): 'No evidence that they do anything beneficial. Some evidence that under certain circumstances, they may cause harm.'