Sleepless PCs and Exhaustion Rules

The rules specifically speak about the need to sleep. If a PC or any character for whatever reason does not sleep, by RAW, why would this rule effect them?

Xanathar’s Guide to Everything gives the optional rule for "Going without a Long Rest"

Going without a Long Rest.
A long rest is never mandatory, but going without sleep does have its consequences. If you want to account for the effects of sleep deprivation on characters and creatures, use these rules.
Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion.
It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.

And the PHB specifies a Long Rest as:

Long Rest
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity-at least 1 hour of walking, fighting, casting spells, or similar adventuring activity-the characters must begin the rest again to gain any benefit from it. At the end of a long rest, a character regains all lost hit points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest. A character can’t benefit from more than one long rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.

If a character des not need to sleep, do these rules effect them?

I understand that sleep is not rest. However, both of these rules specifically state sleeping is a requirement. The question was not is sleep the same as rest.