Let me tell you why numbers matter.
When mechanics eventually get buggered up in a raid, the whole raid will typically take damage. For example, say a pulse from Zuls puddle gets off because someone didnt soak. This shouldn’t necessarily be a deal breaker and an auto wipe. So if the group does wipe, this very obvious large source of damage gets blamed because it puts high strain on the healers to catch back up with the incoming damage. If you can’t as a healer, then you are less important in the raid and you could be kicked from the raid, especially in pugs. If you can’t keep up with the output relative to others, you are more of a liability than an asset.
Heal sniping is an (unavoidable) issue because it feels bad in a raid setting. Example, if you spend 3% of your mana to try and heal someone and lose 75% of the effectiveness because someone else picked it up, you’ve essentially wasted ~2% of your resource pool and potential output. Do that 10 times in a fight and suddenly you’ve lost 20% of your possible output into over healing. This feels even worse if you’ve spent 2 seconds standing stationary only to have to cancel your cast or swallow the overheal because someone else beat you to it. This is really only an issue (if you consider it an issue) with Holy Light and is fairly minor relative to why numbers are important. But the numbers for holy pally output are fine if you can use the kit properly.