3.15 Sequencing: begin, begin0, and begin-for-syntax
The second form applies for begin in an expression position. In that case, the exprs are evaluated in order, and the results are ignored for all but the last expr. The last expr is in tail position with respect to the begin form.
(begin-for-syntax form ...)
expression form expr: converted to (define-values-for-syntax () (begin expr (values))), which effectively evaluates the expression at expansion time and, in the case of a module context, preserves the expression for future visits of the module.
See also module for information about expansion order and partial expansion for begin-for-syntax within a module context. Evaluation of an expr within begin-for-syntax is parameterized to set current-namespace as in let-syntax.