Benefit: use existing code in (existing and new) applications independently of the usage of xPages or native design elements. The goal is to write code once and use it at different places.
Please note (written after first comments) that this idea/request has neither to do with learning Java nor LotusScript. There exists thousands of applications using heavy usage of LotusScript (usually also existing longer than xPage available in Notes/Domino). If you have to modernise such an application it would be a great benefit to use existing libraries written in LotusScript also in xPages (and not to rewrite the code).