In what context?
They do provide strike protection, they’re not used for support.
Would I use boots instead of polos? Yep, less work, much less room for error. But polos have been around for yearrrrrrrrs, long before there were many options for boots, especially fancier boots, so yes there’s a lot of tradition there too.
Lots of horses have some amount of white on some legs, but not even white, and not all 4, so white polos can help un-trick the eye about where a horse is moving symmetrically or not.
You can’t show in them, (there are exceptions), but they can be used in warmup.