The <jsp:useBean> tag (not usebean, case counts) hooks up a scoped variable to a scripting variable. Because scriptlets have been discredited for 10 years now as they were replaced with the JSTL and EL in JSP 2, this tag is used a lot less than it used to be.
There are some arrangements where this tag may cause the scoped variable to be created if it does not already exist, and this side effect is the only really useful part of the the tag when used in modern script-less pages.
P.S. The id attribute names the scoped variable to be targeted and the scripting variable to be created.