Who is this “we”? I consume things I don’t need because I like them; your own decision making process may differ from mine though, so I wouldn’t like to speak for anyone else.
We need very little ; food is cheap, air and water are generally free, shelter is not strictly necessary. I choose to spend money on things I don’t need, such as a warm and comfortable indoor place to sleep, because they bring me more pleasure than the extra dollars in my bank balance might.
“Need” is a pretty high bar to clear, I don’t think that should be your standard for what you buy. I think what you’re asking is something more like “why do people buy things which fail to give them enough benefit to justify their purchase”, which is a good question, but perhaps can only be answered by the individual concerned, through introspection.
Sometimes I worry I’m the opposite — that instead of spending too much money on things that don’t give me benefit I’m being too cheap, and failing to buy things that actually would be worthwhile. On the other hand I’m sure I make errors on both sides.