Why do we need cash?

[Sniper]

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My cash inside of my portfolio is earning some interest, but I'm sure there's better safe investment that earns more interest, like some bonds.

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I'll just note, that you didn't ask what was a better use for the 5% cash... you just asked why portfolio strategists recommend it... and you received several correct answers to that question!

Personally, if you are young, my recommendation is to be fully invested!