If you keep a seemingly useless thing for a long enough time, you will probably have some occasion to use it during that time. The proverb does not teach us to keep useless things for exactly seven years before we get rid of them, but instead it teaches us to be prudent with throwing stuff away as we might need them soon enough, so better keep them around for a little longer.
My husband bought me an useless antique chair for my birthday. What am I supposed to do with it? — complained Anne.
Well, you know, keep it seven years and you’ll find some use for it. — her friend Sarah teased her.
Origin: First written account of the proverb is found on The Palace of Pleasure of William Painter published in 1623.
Things of small value the old proverb say, Wise men seuen yeares will carefully vp lay.