Why does Apple support say you have to water lock the watch everytime or it may break but online it says its only to prevent unintended inputs to the watch?

Here is what Apple states about this topic:

About Apple Watch water resistance - Apple Support

Particularly this: "Water resistance isn't a permanent condition and can diminish over time."

and this: "While the above should be avoided, if your Apple Watch comes in contact accidentally with soaps, shampoos, conditioners, lotions, perfumes, solvents, detergent, acids or acidic foods, insect repellent, sunscreen, oil, hair dye, or any substance other than water, it should be cleaned with fresh warm water and dried with a lint-free cloth. Chemicals found in these items could negatively affect water seals and acoustic membranes."

Water lock locks the touch screen against unintended inputs, and also expels water from the speaker cavity when unlocked.

Water lock does not alter the intrinsic water resistance, or any intrinsic lack of water resistance due to damage or degradation or chemical exposure or otherwise. It is a control lock.

Related:

• How to use Water Lock and eject water from your Apple Watch - Apple Support
• Go for a swim - Apple Support

I have used a Series 5 for pool swimming with no issues. Not past the usual inaccuracies with its exercise measurements, that is.

Because they are human beings, and subject to mistakes.