What project is this for? I assume it's not one of ours, as we've never called for bi-ax in our plans.
As to why I'd use it one something, because it adds far more stiffness than cloth. I'm assuming whatever project this is for, that they called for bi-ax for the structural properties it supplies. On our plans projects, we're only calling for glass as a weather-resistant element (as opposed to personal projects, which have certainly used bi and tri-ax for their strength.)
The knitted fabrics (bi and tri-ax) are a bit harder to get into tight shapes, as compared with woven cloths, so we usually try to dry-fit things as closely as possible before beginning to mix resin. Don't try to dry-fit after you've started the reaction! Time seems to speed way up then...
As I recall, xynole-type fabrics are a good alternative to thin glass cloth as a weather-beating layer. Not much for structure, though, if I remember right.
Mike