Why would a circuit designer use parallel resistors?
Power dissipation will be the driver.

Using six in parallel allows use of standard resistors which may be a stock item.
Using standard parts allows use of automatic assembly equipment.
Lower profile.
Heat spread out over larger area resulting in lower peak temperatures.
Ability to combine to make a non standard value. The 20 Ω in your question is not an E12 value so it's probably not available in wire-wound.
Reliability: if one fails the circuit might continue to operate - but a cascade failure is likely.

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