insurance coverage for orthodontics

[AlexCLE 55]

My kids will need braces and I'm reading up how it's graded on a 1-5 severity scale. If it's 3-5 then it's paid by the statutory insurance we have. I'm also looking at supplementary orthodontic insurance and they also have coverage for KIG 3-5 (for example, 90% up to 2500€). What's confusing is, if statutory insurance already pays for level 3+ why would we even need the extra private insurance? And is 2500€ even worth it to get private insurance? I was under the impression that braces are more like 4-5k€.